So I am building my first D&D character, it's a Barbarian Mountain Dwarf, and I'm trying to figure out what weapons he is proficient with, but in the Barbarian section it says that he his proficient with Simple and Martial weapons. Does that mean he is proficient with all weapons? And if not, then what weapons does it mean?
PHB p. 146:
Your race, class, and feats can grant you proficiency with certain weapons or categories of weapons. The two categories are simple and martial.
A weapon in 5e can be either simple or martial. You can also check the table at p. 149. If you are proficient with both categories, then you are proficient with all weapons (except unconventional improvised weapons, which need the Tavern Brawler feat in order for you to be proficient).
It does, but only in weapons from the PHB
Simple and martial weapons are the only two categories described in the PHB in the Chapter 5: Equipment. However, Monster Manual (and Volo's Guide as well) describes creatures using various different weapons, out of the PHB list.
Some examples are:
- A Grimlock's spiked bone club
- A Kuo-toa's pincer staff
- A Redcap's sickle
- A Chain Devil's chain
From the lore perspective, it is highly unlikely that your character will be proficient with all these weapons, and in terms of the game mechanics they are neither 'simple' nor 'martial'.