In this menu, users are able to perform recut/offset operations to the subtitles. These can be done in two ways.
The first one is by Index. This simply means that the user selects the first subtitle he wants to recut/offset and the last one, therefore creating an interval.
The second one is by Time. The user can, similarly to the first option, define the interval to recut/offset by selecting the first in cue and the last out cue to be offset.
So, basically, the recut option allows you to use previously created subtitles. The subtitles were probably created for a video with a different timecode and now the user wants to adapt them to the same video with a new timecode, without having to spot the whole video again.
Offsetting is a bit different. If for some reason, during the spotting process, all or some of the subtitles become unsynchronized, the user can move the selected subtitles forward or backwards at the same time.