Once you get to level 70, the level cap, you have the option to start Veteran Mode. Basically, you start from scratch, but you get a few perks to match. Here is a list of what happens when you enter veteran mode.
You start over at level 1, and lose all skill points, and what you have purchased with those skill points, but you keep your medals, ribbons, and stats.
You get the chance to play on another PMC if you wish to do so (This is not required, as you can just stay in the PMC you are already in if you want to, but this is one way you can earn all of the gold lvl 60 trophies).
You get a permanent 10% Experience bonus for all future matches. This bonus is not cumulative, meaning you receive a 10% bonus the first time you enter Veteran Mode, as well as the second time and each consecutive time.
You earn a gold symbol with the number of times you have entered Veteran Mode below your rank.
Also, please note that veteran mode can be done more than once. If you want to join a new PMC for the moment, you don't have to stay with them for good.