Double time isn't genre-specific, it's a music theory thing. It just means that you have a part in a song that sounds like it's at double the song's regular tempo - so in an 80bpm track you'd have a part that sounds like it's at 160bpm.
But in terms of sequencers - and step sequencers (which a piano roll essentially is as well) especially - it's often used to mean that even though you're writing a song @ 80bpm, you'd set your sequencer's tempo at 160bpm, so if you'd regularly have 16 steps in a bar, you now have 32 steps, so you can make quicker hihat fills, for example. Of course your sequence also needs to be double the length as well in this scenario. This "trick" stems mainly from old hardware sequencers that had limited resolution, but of course in modern DAW piano roll environments it's not really necessary, as most piano rolls allow for practically infinite resolution for what it's worth (well, maybe not infinite...but I doubt you're going to be needing to be more precise than 1/16384th notes which is the smallest as Ableton will go, for example).