60i (or more accuratly 59.94i) basically means 60 frames of motion crammed into a 30fps signal. The odd lines are recorded first then half a frame later the even lines are recorded and so on and so forth. thanks to magic science stuff that was worked out back in the 50s.

Interlacing doesn’t work well on web video so upscaling a 60i to a 60p preserves all 60 half frames of motion as full frames, giving that nice smooth motion effect. This is what it would have looked like on CRT TVs back in the day.