org-clock-in is an interactive autoloaded Lisp function in `org-clock.el'.

(org-clock-in &optional SELECT START-TIME)

Start the clock on the current item.
If necessary, clock-out of the currently active clock.
With a prefix argument SELECT (C-u), offer a list of recently clocked
tasks to clock into. When SELECT is C-u C-u, clock into the current task
and mark it as the default task, a special task that will always be offered
in the clocking selection, associated with the letter `d'.
When SELECT is C-u C-u C-u, clock in by using the last clock-out
time as the start time (see `org-clock-continuously' to
make this the default behavior.)