Citrus rum is a flavored rum that you can think of as the rum equivalent to the ubiquitous Absolut Citron. It is commonly found in rather simple, sweet drink recipes. Quality is usually pretty low for citrus rum though Cruzan makes a decent one.
As always if you are planted in the liquor store sifting through two or three varieties of citrus rum, try the strongest one first. This is not a guarantee of quality but at least you have some good chance of getting a more balanced citrus rum if the proof level is in the fifties versus a thirty or forty proof cloying alternative.