Founder vesting subjects the founders' equity to a vesting schedule (often 4 years with a 1-year cliff, sometimes with credit for time already served). Without founder vesting, a co-founder who leaves after 6 months would walk with their full equity, leaving the remaining team and investors holding the bag.
Founder vesting is standard in VC-backed companies. It's typically renegotiated at each priced round, and credit for time already served is the most negotiated element. Founders should establish vesting before the cap table gets complex.