First, they will want to make sure that you are safe and then talk with you about why you went and what happened while you were away.
The idea of speaking to police and social services can seem daunting and you might not feel ready. Try to make it a two way chat; if you want a bit of time then make sure they know they can call you in a couple of days.
In some cases they might be concerned about the situation you are going back to and want to get some more information. It might be, for example, that they think you ran away because you were unsafe at home. If this is the case, they will need to know more before they decide what to do next. There is a chance they might ask for what is called a Police Protection Order. The order lets the police keep you with them in place of safety (this can sometimes be temporary foster care) while they get the information they need.
Remember, this is only used in cases where there is a lot of concern; mostly they will simply want to check that you are safe and well.