Resumable rendering is the ability to have incomplete renders resume where they left off from the previous session.The rendering could have stopped because of external forces, such as a power failure or having the job bumped on a render farm, or when you stopped the render manually and want to start from where you left off later.
Example Video of the features from V-Ray for Maya
https://www.youtube.com/watch?v=6_E5ZMlE_eo