Resnap refers to a pair of commands under the Timing tab in the beatmap editor. One of which applies to the entire beatmap and the other to the current timing section. If the beatmap has only one timing section, both commands are the same.
This command will automatically reposition all hit objects on the timeline so that they fall on the closest beat division that the snap divisor allows. Usage is suggested when there has been a change in offset and/or BPM, after objects have been placed. Many creators recommend checking every hit object and adjusting its position on the timeline manually due to this tool's inaccurate and systematic nature.