It's slang for 'abdominal muscles'.

"I'd like to see his abs" is used when you think another person has a fantastic body. Usually, a person would say this if they are interested in the person romantically but don't know them very well. We would like to see their six pack (sexy abdominals) We could also say this about a person who says they are fit but we doubt them. We think they aren't fit, so we'd like to see their abs to prove that they don't have a six pack.