Metronomes are tools to keep tempo, giving and audible click or beep at the set rate. They are available in wind-up mechanical varieties that click, or newer electronic models that beep. Each is frequently used by students and teachers alike to keep in time with music.
An alternative to a metronome is the click track, which is essentially a recorded track of a metronome. Some critics of the click track claim that it takes the human sound of the music away, leaving it feeling lifeless and mechanical. Keep in mind that a professional drummer is often likely to be just as accurate as a click track, and that without a steady beat it would be more difficult to be expressive through slight variations in tempo.
Also, if you don't like the feel or sound of a click track, most DAWs (including Reaper) will let you replace the audio with something much more musical than the tick-tock or cleep-clop of a click track, like actual drum hits.
See our article on how to use both a click track and/or a metronome in Reaper here: Using a Click Track For Recording Music