to follow someone and watch what they do, where they go etc:
The police have been tailing him for several months.
tail awayphrasal verb
tail backphrasal verb
if traffic tails back, a long line of cars forms, for example because the road is blocked
tail offphrasal verb
to become gradually less, smaller etc, and often stop or disappear completely:
Profits tailed off towards the end of the year.
if someone's voice tails off, it becomes quieter and then stops:
'I didn't mean ...' Her voice tailed off in embarrassment.