Hardy fish is a term which refers to any fish which is easy to maintain in home aquaria due to their ability to adapt to a wide range of water parameters. Such fish are good starter fish and ideal for 'cycling' your aquarium (ie. reducing ammonia and nitrites to effectively zero.)
Examples of hardy freshwater fish include Poecilids such as the guppy, molly and platy, most species of Corydoras and some tetras. Generally, fish are hardy only when they have been tank bred as wild-caught species, since fish take many generations to adapt to life in captivity. Look online for a fish you like. I f the water parameters look anything close to this:
pH:6-8
Temp:68-80
You should be okay.