As a new player to D&D, I don't really know what significant changes have been made to the Ranger through Unearthed Arcana. What makes the class different from the PHB version and how does this help make the ranger more viable?
This article does an excellent job of describing in detail the changes between the stock ranger and the UA Ranger Revised, but I'll summarise for the Stack. Generally the Ranger is made more mobile, its abilities more widely applicable, and attack more dangerous, but probably the most important change is the introduction of the Beast Conclave archetype, as a direct redesign and improvement to the Beast Master archetype.
The base class changes include:
- Favoured Enemy: Adds a +2 damage bonus, condenses humanoids into a single type, initial choice limited to a selection of less powerful enemy types. Increases to a +4 damage bonus at 6th and offers the choice of a greater favoured enemy from a list of more powerful enemy types, with the extra bonus of advantage on saving throws against effects produced by the Greater Favoured Enemy. Added damage against favoured enemies increases the Ranger's combat effectiveness nicely, and advantage on saves against certain powerful enemy types comes in very handy.
- Natural Explorer: Eliminated double proficiency to Intelligence and Wisdom checks in relation to favoured terrain, but other benefits now apply pretty much all the time regardless of environment. Plus, offers several combat advantages - ignore difficult terrain, gain advantage to initiative, and advantage against slower enemies in the first round of combat. Ranger is now more generally useful in all terrains, plus the combat bonuses help them start the first round on the right foot.
- Primeval Awareness: Removed dependency on a limited resource (no longer needs a spell slot), made more powerful and precise - can detect enemies up to five miles away and know the exact number and distance/direction. Also allows you to commune with animals.
- Fleet of Foot: Grants ability to Dash as a bonus action, which is generally useful for anyone anyway, but contributes to the ranger's mobility.
- Hide In Plain Sight: Now just causes a -10 penalty to enemy perception checks to spot you so long as you don't move. Has utility in combat and is easier to take advantage of than the PHB version.
The Beast Conclave archetype works drastically differently to the original Beast Master archetype, which is great, because Beast Master Rangers are the single worst class in the core PHB. The improvements are many:
- Action economy: The animal companion now acts on its own turn in the initiative order and doesn't require sacrificing the Ranger's actions to command to do anything, so the Ranger has much more versatility in combat.
- Defence: Proficiency in all saving throws, upgrading to also gaining advantage on saving throws while it can see the Ranger. Adds proficiency bonus to AC, and eventually gets Uncanny Dodge. Gains hit points based on hit dice so probably works out to having more HP than the PHB equivalent.
- Attack: Add proficiency to damage. Explicitly lose multiattack from the base creature, but quickly gains ability to make extra attacks with its Reaction (when the Ranger uses the Attack action) and eventually Storm of Claws and Fangs allows it to attack all adjacent foes in a single action.
- Utility: Uses the Ranger's proficiency bonus instead of its own, and gains proficiency in two extra skills. Also gets the benefits of the Ranger's Favoured Enemy feature and can move stealthily alongside its master using Natural Explorer. Gets an ability score improvement whenever the Ranger does.
The other new archetype from the Ranger Revised, "Deep Stalker", made it out of UA into published status in Xanathar's Guide to Everything as the "Gloom Stalker", so I won't discuss it here.
Essentially, the Ranger Revised UA material presents a ranger with more general utility in all circumstances, abilities that support it being an amusher/skirmisher with enhanced mobility, more dangerous attacks and also the option to play the traditional ranger archetype with an animal companion that isn't an underpowered mess.
The biggest change is that Favored Enemy applies to damaging those foes.
There are also a number of utility abilities that allow a group to travel further, faster, and safer when a Ranger is in the party (as opposed to an Outlander Fighter). While these are useful, they are less significant to players than the damage increase. (They may be more significant to the campaign, but that requires meta knowledge that players rarely have.)