Interva training is exercise that has breaks between periods of exercise. i.e. ride for 5 minutes slow. Stop for 1 minutes, 5 mins medium, stop, etc.
ideally don't 'stop' between intervals as above, but (for cycling) pedal easy instead.
and no, you don't need to do 90 mins for an interval session.
a nice one is the '10 to 2 descending ladder'
progressive warmup (15 mins or so)
10 minutes at threshold
2 mins easy
8 minutes at threshold
2 mins easy
6 mins at threshold
2 mins easy
4 mins at threshold
2 mins easy
2 mins at threshold
10 mins warmdown.
total just over an hour.
or…taking it to extremes – google 'tabata' and meet my friend pain.