The first time you attend a Zoom meeting on a Mac, you'll need to download the Zoom app. Here's how to do that. (Zoom also has instructions for connecting from a MacOS device.)

You'll start by clicking the meeting link, either in an email or on a web page (depending on how you received it). That will open a Zoom page in your browser, like this (Safari shown here):

Unless you are the host, you'll click the "Join Meeting as an Attendee" link below the button, which will take you to this page.
Click "download & run Zoom." That will start a download (Safari shows an animation) and a file called "Zoom.pkg" will be downloaded and will appear in your Downloads folder or desktop (or elsewhere, depending on where your browser is set to store downloads). It looks like this:

Double-click it to launch the installer, which will put up the typical Mac installer window.

Click "Continue" on this screen, then "Install" on the next. You'll be prompted to enter your password or use Touch ID to authorize the installation during the process. Once it's done, the installer will show "The installation was completed successfully" and the Zoom application will start up (which might cover up part of the installer, as here):

You can quit the installer whenever it's complete. Click on the browser window that was first opened (the backmost window in the above) to bring it to the front:

Now you can click on the "click here to join the meeting" link. The browser will ask, "Do you want to allow this page to open "zoom.us.app"?

Click "Allow." This will bring the app back to the front, and will bring up a small dialog asking you to enter your name.

Enter your name (note that you can check a box to have Zoom remember it), then click "Join."

You'll be presented with a small window to choose whether to use your computer's speaker and microphone. In most cases, this is best, so you'll click "Join with Computer Audio."

And you're in!

There are controls across the bottom. The two most important are at bottom left: "Mute" to turn your microphone off (it's good etiquette to keep it off when you're not talking, to prevent background noise from intruding on others), and "Start Video" (referring to the video of you; this is normally off to begin with, and you may wish to leave it off to reduce bandwidth requirements).

Happy videoconferencing and happy folding!