Level and experience isn't too confusing once you get used to it.
Every character starts with 15 BP (Build points) These are spent to buy whatever skills you would like to have as per the Build table a bit farther in the book.
Every event you attend as an NPC or PC you gain XP equal to your build, so a new player at a one day event would gain 15 XP (as they have 15 build) that experience is then turned into build depending on your level, at level on its 3 XP per build, so you'd gain 5 build (to a total of 20) which you can then use to add skills to your character, perhaps picking up thrown weapons that they learned the last game, or a critical attack. As you go up in levels the additional build you gain per event day slowly goes down (hitting roughly 1 per event day around level 16)
"Levels" are purely an Out of Game measure of how big characters are, you level can be found by taking your build, minus 5, divided by 10 rounded down. We use levels when preparing for events to determine how big the monsters should/can be, as well as the amount of treasure that plot will put out.
A Build Cap is entirely differnt, and is usually for newer campaigns what a build cap is, is a restriction on how big of a character can be played at an event. This is done generally to help a new campaign build up their local base, as well as let the staff get better at running things, without having to stat/prepare for long played characters (not this is a character restriction, not a player restriction, and we have many travelers with 'alts' just for such situations) These caps are usually removed/opened up within a few years.