What is "bookending"

Usage examples of "bookending".

Dennis crossed the parking lot carefully on his crutches, his mother and father bookending him nervously in spite of the fact that the lot had been scrupulously salted free of even the slightest trace or snow and ice He paused by the family car, turning his face up slightly into the freshening breeze.

I watched him stroll into fancy restaurants dressed in a tuxedo, always with a different woman on his arm, always with his pair of bodyguards bookending him.