A campaign is a canvas of action sets connected by lines and, where the logic needs to branch, decision diamonds. An action set is a small bundle of steps, send this email, wait two days, apply that tag, notify a rep, and a decision diamond checks a condition, did they open the last email, do they have a certain tag, and routes the contact down one path or the other depending on the answer.
If your account goes back to the Infusionsoft era, you may remember an older version of this canvas. Keap has continued to update the builder's interface since the rebrand, which is why searches for the newer layout show up alongside tutorials written against the older one. The underlying idea, action sets and decision diamonds strung together on a canvas, hasn't changed.
Campaigns also carry goals, the specific finish lines a contact can hit that move them into the next stage of logic, which is how Keap avoids forcing every contact through the exact same linear path regardless of what they actually do.