One addition to the comments so far - if you install an add-on from a zip then it's a "one-off". You'll just get that add-on, and that particular version of the add-on.
If you install a repo, then you can get any of the add-ons that are contained in that repo (many authors have multiple add-ons all contained in a single repo). And also more usefully if they update the add-on and push that update to their repo, then you will also get that update (if you have auto-updating enabled on your Kodi set-up). Hence bug-fixes and new features will arrive without you having to do anything.
Also some add-ons have dependencies (other add-ons or parts thereof which they need to run). In most cases authors who use repos will include those dependencies in the repo (if they are not commonly available from the official repo or similar). If you have the repo installed then they will be added automatically, but if you just install the add-on from a zip then you may need to go and find the dependencies yourself and manually add them.
The built-in official Kodi repo is an example, but as @
trogggy said above you can add other author's 3rd party repo's by downloading the repo zip file and adding it. Then all the repo content becomes available via the install from repo option.
One small caveat though. It may be worth a quick read of
this blog to make yourself aware of some of the less desirable aspects of certain 3rd party repos and what they may contain.