Vesting ties equity ownership to continued service. The standard schedule for founders, employees, and most advisors is 4 years with a 1-year cliff: nothing vests for the first year, then 25% vests on the cliff, and the remaining 75% vests monthly over the following 36 months.
Vesting protects the company (and the team) from a co-founder or early hire walking away with full equity after a few months. It also creates retention pressure as unvested equity grows in value.