Hook up fees are the fees that you (the tenant) pay to have your utilities activated. They're also referred to as installation fees. Almost all utility companies will charge you to activate their services.
These fees are a surprise to most people, yet they exist nevertheless. Don't let them come as a surprise and make sure to save up so that you can pay them immediately.
Unfortunately, these fees can't usually be negotiated. Utility companies charge you when you begin using their services, plain and simple. Sometimes, if you've had a very difficult time with the utility company or they've given you a hard time, you might be able to get these fees waived. But, that's not too common.
You should become aware of these expenses so that you can save additional money in preparation for moving into your new apartment.
They're usually not too high in price, so you won't get an outrageous bill that comes out of nowhere. Regardless, you should make it a point to contact the utility companies before you move into your new apartment and ask how much the fees will be so that you can prepare to pay for the amount in advance.