Using abstract nouns, adjectives, adverbs, et al.

[Jezie]

Ok ok... This may get me into trouble... Yet... I am stuck. Abstract nouns, easy enough... But why ponder on abstract nouns and not catch the "they're
use can usually be attributed to a fit of sloth. ..." maybe it is because I break contractions up when I read and do not use them when I write. But this line bugs me... I do not know who Mary Oliver is... But why was "they're" used and not "their"? Again, not meaning to be rude or anything, just curious on basic grammar... And what I am missing here...