An ENG-GER dictionary (there seem to be no entries in monolingual dictionaries) tells me that it is a power connection, but search results from Google and Google Images let me assume that the term is used only for outdoor purposes, and especially for camping sites. Is this correct or what else would be an appropriate meaning?
According to Cambridge online dictionary, hook-up means:
a connection to an electricity supply, a telephone network, the internet, etc.
In North American English variants, hook-up typically means the connection to any utility, irrespective of where. The meaning is very generic. Your house might get a new telephone hook-up, or Rogers might be hooking up your new Internet.
I wish my home Internet hook-up was as fast as the power at our campsite: took them half an hour!
"electric(al) hook-up" often refers to the hook-up cable (basically a wire) with which you can connect your unit to a bollard (power bollard or electrical distribution bollard) or a utility post. Either end of the connection is called a hook-up point. Sometimes "electric(al) hook-up" refers to the action, like in, "I've established an electrical hook-up to my computer". (Camping and caravanning club)