When a musician records a musical part on a track in a DAW, he listens to the metronome click and plays along with that, using those regular clicks as a reference so the notes he plays/records will be in time with the bar/beat ruler in the DAW. The metronome clicks represent beats, which are grouped together into a repeating pattern called a bar (or measure), with an accented beat (click) followed by several unaccented beats (clicks). The pattern of accents is based on the Time Signature -- so in 4/4 time, there will be 4 beats (clicks) in each bar (each group of metronome clicks), one accented beat (click), the downbeat, plus three unaccented beats (clicks), the rest of the beats in that bar. Musical parts/rhythms are played along with the metronome clicks -- this makes it easier to play/record multiple tracks and keep them in time with each other and with the DAW bear/beat ruler. If the musical parts (notes) are in time with the DAW bar/beat ruler it makes it easier to make musical edits like cutting and re-arranging Regions or quantizing notes for tighter musical timing.
To explain music theory properly really requires audio examples, which cannot be included in a text-based discussion group. I encourage you to watch the recommended videos -- I've attached a different course to this thread -- watch the 1st video and then skip to video # 6 and watch (& listen ) from there for a fuller explanation of how music theory of rhythm applies to DAWs:
And here's the link to the previously-attached music theory course, in case you neglected to bookmark it: