Eloise,

These are sins, not in bad deeds that you do, but in good deeds you don't do.

For example, the other day I saw a woman in a mall who was clearly homeless and needed new shoes.

My conscience told me I should approach her and offer to buy her new shoes. But I failed to follow that inspiration, because (in part) I had a movie to go to with some friends.

That would be a sin of omission: I "should have done something but didn't".

These are in contrast to sins of commission, which is when you "do what you aren't supposed to do".