What is an interval in an F1 race? How long does it last? How can it be activated and deactivated?

An interval can be described as the gap between a pair of cars measured in time.

i haven't seen it before

What do you mean by intervals? Do you mean tyre stints in which case it's how long a car is on each tyre defined by tyre wear. Do you mean timing sectors on each track? In which case the track is divided up into 3 sectors evenly split across the track. This is then broken down into further sectors (generally by Marshall posts).

Do you mean DRS?

I don't understand what you are asking - the only 'interval' in F1 racing when watching on a commercial channel, is a break for the commercials.  Which is why I tend to record Quali and the Race so I can fast forward through the ads.  I suppose that's a form of 'deactivation'?

a pre defined section of track sometimes also referred to as a sector

most tracks for timing purposes,   are split into 3 generally equal lengths of track...and a timing sensor under the track records car timing data...to inform race strategists...and also helps with informing the spectator or other audience   ie  tv  etc