The word "interval" means: an intervening of time and a pause or break in activity. With interval recording you can set the camera to record 1 second of video every 30 seconds, or 1/2 a second of video every 15 seconds, and so on. The effect when played back is fast motion video.
You can also choose to film individual frames of video manually. This is useful for making animation.
On the PAL HVX, the interval can be set so that a single frames are taken with
2 frame - results in footage that speeds up time 3 times
4 frame
8 frame
16 frame
1 second
2 second
5 second
10 second
30 second
1 minute
5 minute
and 10 minute (15,000 frames - time is sped up 15,001 times, a day goes by in less than 6 seconds)
'intervals' between.
You can only take a single frame for each interval, unlike other cameras which give you the option of taking differing amounts of video between intervals.