Contingency is an event that depends upon another event or events for its occurrence. Therefore, that which is contingent may or may not occur, depending on what happens. It is also an alternate plan in case a previous expected event does not occur.
In Christianity, God is non-contingent in that he does not need anything else for his existence or decisions. We know this because he is the creator of all things (Isaiah 44:24). If he is the creator of all things then he does not depend on what is created for his existence since he was existing without them.