"Tapping" or "tap-to-click" is the name given to the behavior where a short finger touch down/up sequence maps into a button click.

This is most commonly used on touchpads, but may be available on other devices.

libinput implements tapping for one, two, and three fingers, where supported by the hardware, and maps those taps into a left, right, and middle button click, respectively. Not all devices support three fingers, libinput will support tapping up to whatever is supported by the hardware. libinput does not support four-finger taps or any tapping with more than four fingers, even though some hardware can distinguish between that many fingers.

Tapping is disabled by default, see this commit because:

if you don't know that tapping is a thing (or enabled by default), you get spurious button events that make the desktop feel buggy.
if you do know what tapping is and you want it, you usually know where to enable it, or at least you can search for it.

Tapping can be enabled on a per-device basis. See libinput_device_config_tap_set_enabled() for details.

Tap-and-drag

libinput also supports "tap-and-drag" where a tap immediately followed by a finger down and that finger being held down emulates a button press. Moving the finger around can thus drag the selected item on the screen. Lifting the finger and putting it back down immediately (i.e. within the timeout) will continue the dragging process, so that multiple touchpad-widths of distance can be covered easily. If two-fingers are supported by the hardware, a second finger can be used to drag while the first is held in-place.

Constraints while tapping

A couple of constraints apply to the contact to be converted into a press, the most common ones are:

the touch down and touch up must happen within an implementation-defined timeout
if a finger moves more than an implementation-defined distance while in contact, it's not a tap
tapping within clickpad software buttons may not trigger an event
a tap not meeting required pressure thresholds can be ignored as accidental touch
a tap exceeding certain pressure thresholds can be ignored (see Palm detection)
a tap on the edges of the touchpad can usually be ignored (see Palm detection)