Hit points and Hit Dice are explained in the Basic Rules which you can download for free from Wizards of the Coast web site. (Page 7 of the Basic Rules explains this).
- Note: You state that you are new to D&D. If you are reading a players guide before downloading the Basic Rules, and are new to the game, you are making a mistake. Download the Basic Rules to get an idea of what it all means before trying to create that Monk.**
Hit Points and Hit Dice (p. 7 Basic Rules)
Your character’s hit points define how tough your character is in combat and other dangerous situations. Your hit points are determined by your Hit Dice (short for Hit Point Dice). At 1st level, your character has 1 Hit Die, and the die type is determined by your class. You start with hit points equal to the highest roll of that die, as indicated in your class description. You also add your Constitution modifier, which you’ll determine in step 3.) This is also your hit point maximum. Record your character’s hit points on your character sheet. Also record the type of Hit Die your character uses and the number of Hit Dice you have.
Hit Dice: the dice used to add up all of the creature's hit points
Hit points: an indication of how healthy or how close to death a creature is in the D&D game.
Hit points represent a combination of physical and mental durability,
the will to live, and luck. (Basic Rules(2018), p. 77)
How are they used in combat? (Easy step-by-step)
When a hit is scored in combat (by rolling a high enough score with a d20), a damage die is rolled and the amount of damage is subtracted from the old hit point total to create a new hit point total. As long as a creature's hit point total is a positive number, it is alive. Once it reaches zero, it is either dead or unconscious.
- Example: your monk rolls a 17 with a d20 and the DM tells you that it
is a successful attack. You roll a 1d4 since you were using a
Martial Arts attack, and a 3 is the result. You add 3 more due to
your dexterity bonus of +3 for a 16 Dexterity: the total damage is 6. The DM subtracts 6 from the creature's hit points. If the old hit point total was 7, the creature is alive. If it was 6 or less, it falls, dead or unconscious, as a result of taking that many hit points of damage.
For Monk in the guide it says Martial Arts LVL 1: 1d4. Does this relate to hit points and die?
No, that is related to how much damage the Monk does when using the Martial Arts skill. As above, the Monk has to first roll a "hit" with a d20 before rolling the d4 to see how much damage he did with a successful attack.