The root "gress" is associated with motion (for example egress or retrogress). However, the word "gress" is not found in an English dictionary, so "gress" is a bound morpheme.
To identify the root morpheme of "ambidexterity," remove the prefixes and suffix:
The resulting root is dexter; a dictionary reveals that this is not an English word. Therefore dexter is a bound morpheme.
However, "freeze" in "antifreeze" is a free (non-bound) morpheme.
The root here is freeze; a dictionary will confirm that this is its own word. Therefore freeze is a free morpheme.
Affixes are themselves bound morphemes because they are used only when attached to a root morpheme and/or another bound morpheme.
Common English bound morphemes include: -ing, -ed, -er, and pre-.