A MIDI event that carries the MIDI clock message keeps a slave MIDI device synchronized with a master MIDI device. The MIDI clock message is a timing message that the master device sends at regular intervals to tell the slave device where it is in terms of time.
This message belongs to the category of MIDI system realtime messages.
The MIDI clock message consists of a single byte of data. This byte is the status byte and has the value 0xF8.
According to the MIDI protocol there are always 24 clock ticks per quarter note. How long a quarter note is depends on other MIDI messages (see MIDI Set Tempo meta message and Time division of a MIDI file). Thus, the time between two clock ticks varies and the MIDI clock ticks have nothing to do with the MIDI time code. Also, the frequency of the clock ticks has nothing to do with the frequency of internal MIDI device clock ticks or with the ticks that form the MIDI delta time in MIDI events (the resolution of the MIDI sequence; see MIDI event).
With 24 MIDI clocks per quarter note, the master will send one message for each clock. Thus, the slave device will know that a quarter note has passed after 24 MIDI clock messages.