Stashing refers to the practice of adding a player to your team with the intent of holding them for the future. Injured players, rookies, and backups are the most commonly stashed players.
Using the 2022-23 season as an example, Jaren Jackson Jr, who was recovering from a serious foot injury, was a player who was commonly stashed. Prior to the season, fantasy owners drafted Jackson with the knowledge that he would likely miss the first 1 to 3 months of the season. Because of that, he was able to be drafted much later than normal. Owners had to "stash" Jackson, either on their bench or with an Injured Reserve (IR) spot, until he was cleared to play.