Meaning of “tap” - Learner’s Dictionary

tap

verb uk us /tæp/ (present participle tapping, past tense and past participle tapped)
KNOCK [ I, T ]

B2 to knock or touch something gently:

She tapped her fingers nervously on the table.
I tapped on the window to try and get her attention.
A SUPPLY [ T ]

If you tap a supply of something, you use what is available:

There are immense natural resources here waiting to be tapped.
TELEPHONE [ T ]

to use a special piece of equipment to listen to someone's telephone calls:

[ often passive ] I think the phone's been tapped.

(Definition of “tap verb” from the Cambridge Learner’s Dictionary © Cambridge University Press)