What does "be my guest" mean?

Do as you wish. For example, May I drive your car?—Sure, be my guest, or Do you mind if I go to the play without you?—No, be my guest. This expression not only literally invites someone to behave as one's guest (using one's house, belongings, etc.) but also figuratively tells someone to feel free to act as he or she pleases. [Colloquial; c. 1950] Also see feel free.