Every pending beatmap has a hype train that roughly represents how many people are interested in seeing the map ranked. Users can post special beatmap discussions in the General (All difficulties) tab with the Hype! option to contribute to a map's hype train.
Beatmaps need at least 5 hype (one complete bar of the hype train) to be eligible for nomination, a step towards getting the map ranked.
Users start with 10 hypes, and they are consumed when used on beatmaps. Each hype takes a week to become available again.