It's a more forceful and direct way of saying "get moving" or "get up and do something." It implies that someone has been inactive for too long and needs to take action.
Here are some examples of how you might use "bestir yourself":
* "It's getting late, so bestir yourself and get ready for bed."
* "The deadline is approaching, so you better bestir yourself and get to work."
* "Come on, bestir yourself! The party's about to start!"
The word "bestir" is somewhat archaic, but it still has a strong and vivid meaning in modern English.