Rebuild is an array operation that recreates all missing data in an array onto spare space, thus increasing it’s failure tolerance. You can think of a rebuild as performing reconstruct operations on the all the missing data, and writing the reconstructed data to the spare space. In arrays with multiple failure tolerance and experiencing multiple failures, rebuild can refer to each handling failure individually, or to handling them all.
For example, in RAID 6, which has dual failure tolerance, experiences two failures leaves the array unprotected. A first rebuild can be performed onto spare space, bringing the array up to single failure tolerance. A second rebuild can be performed onto further spare space, restoring the array to dual failure tolerance. Thus, we can speak of rebuild for each of these operations, or for both combined.