Typically reserved for founders, this indicates shares are owned immediately when granted but subject to repurchase as defined by the vesting schedule. This can also be referred to as a repurchase right.
A founder is granted 1,000 shares of common stock with reverse vesting on a 4 year term, 25% cliff after the first year and monthly vesting thereafter.
After the 12 months, the founder leaves. Because reverse vesting was in place, the company has the right to repurchase the 750 shares the founder had yet to vest.