There are many definitions for the word slip. Here are a few of the most common expressions for the verb from.
#1) My English skills are slipping. = My English skills are getting worse.
Sample Situation: You used to be proud of your English skills, but now you find it hard to communicate.
#2) It slipped my mind. = I knew about it, but I forgot (temporarily).
Sample Situation: You had an appointment with your friends, but you completely forgot about it and didn't go.
#3) I slipped up. = I made a mistake.
Sample Situation: You were at a party and you tried to kiss your girlfriend's sister.
#4) It just slipped out. = I said it on accident.
Sample Situation: When you argued with your girlfriend, you told your girlfriend to shut up, perhaps adding a swear word or two. You quickly regret saying it.
