node executes itself in corresponding thread when someone calls it (another node, loop or user from patch editor).
when executes node works with input data (upper pins) and produces output data (lower pins), and can call following nodes. by default node call following node on pin 0 (default trigger pin).
trigger link is used to indicate what next node to call. color of trigger pins and link corresponds to actual thread. under the hood its a data link but with no data.
in this example order of nodes calls is following:
number_io (sets 0 on data link) -> sleep (stops thread for n seconds (default is 1)) -> print_number (prints number from number_io)