Not a silly question, but judging from your responses, I think this may be above your current level of language learning.
'Undoing' is a verb form acting as a noun-- see 'gerund' in your grammar book or in on-line grammars.
'Their' is a plural pronoun, so it cannot refer to 'bear', which is singular.
See those terms, and also 'predicate' in your grammar book as well.