"Label" — if I understand GP's point — is just a random string you attach to an item. "Amsterdam" could be a label, and it can be attached to a PDF of an old map of the city of Amsterdam, to a photo of your neighbor's dog named Amsterdam, or to an expense report for the project you're building on Amsterdam street. You decide what to attach it to, and it's only important what "Amsterdam" means
to you. The search for "Amsterdam" label would bring you all of the above.
"Tag" — if I understand GP's point — implies some structure. In the above examples, it would rather be "ByLocation::Planet Earth::Europe::Netherlands::Amsterdam", "BySubject::Animals::Dogs::Amsterdam(MyNeighborsDog)", and "MyProjects::ByYear::2021::ProjectOnAmsterdamStreet" — or similar. In this case, if you're interested in the neighbor's dog, you're searching by its specific tag, or — if you don't remember its name — a search for "BySubject::Animals::Dogs" might help you. Any such search will also keep the other "Amsterdam" results away.
The problem with tags is how exactly do you implement them. Your project on Amsterdam street may have begun in 2021, but still ongoing in 2022 with no end in sight; or you may have forgotten its dates at all. Additionally, "MyProjects::ByLocation::MyCity::AmsterdamStreet" is not an invalid way to tag that project.