To be 'up against' something can mean to be close to something or to be in contact with it. 'Up against' can also mean to be confronted with something unpleasant. If you are 'up against it' or 'up against' something it can also mean that you are in a difficult situation. This is a common expression in social and professional contexts and is similar in meaning to 'contending with' and 'facing difficulty'.

1. I went to watch the rival team before we played them so that I could see exactly what we're up against.

1. Two members of staff have called in sick and the restaurant is fully booked, so we're really going to be up against it this evening!
2. I'm very stressed at the moment because I'm up against a deadline and still have a lot of work to get through.