The rhythm of modular lives in gate and trigger pulses. A single Clock flashes at tempo, and the rest of the rack listens.
A gate is a square pulse. On or off. The Clock module fires one every beat.
On the scope you see the clock pulse: a flat bottom, a fast rise, a flat top, a fast fall. That shape is how modular says "now" to the rest of the rack.
A trigger is the same thing but shorter - a quick blip instead of a held-high pulse. Both are used to start envelopes, advance sequencers, fire drum hits.