The preferred term now is “backlog refinement” due in part to the connotations of the word “grooming” in other countries (such as the UK).

Backlog refinement is the practice of reviewing the future backlog items with the team to ensure that upcoming work is understood, estimated, and broken down into chunks that can be completed in a sprint. It is an essential part of the process that when done properly significantly reduces the time spent in sprint planning and increases the team’s overall understanding of the broader scope of work that they are engaged in.

It’s an early opportunity for the team to identify work that is unclear, acceptance criteria that may be difficult to achieve, questions they have about future work, and to ensure that upcoming work is sized appropriately.

I generally find that having a one- or two-hour backlog grooming once per sprint (somewhere in the middle) tends to be sufficient to cut my teams’ planning sessions in half or more, as we have dedicated other time to answer questions and address any issues before we plan the next sprint.