A guide finger is a finger that

is not needed to play a chord

maintains contact with a string when the left hand is moving up or down the fretboard

maintains contact with the metronome when you change between two chords

remains in the same place when you change between two chords

The correct answer here is the last option.

A guide finger, which is also called a pivot finger, is the one that remains in the same place when you are changing between to different chords. are extremely important as the provide a physical "vehicle" so you can jump from one position to another position, while also making sure that your finger does not move to another string.