QGIS is a full-featured open-source GIS application. The download instructions vary a little depending on which platform and operating system you're using. For both platforms, you may want to choose the "long term release" version (currently 3.22): it's been out a little longer and has fewer late-breaking glitches.
Choose QGIS Standalone Installer Version 3.22 (or later). It's the second download on the page. The first download is "QGIS in OSGeo4W", which has some proprietary modules and is tricky to install. Skip it and go with the standalone version.
Choose QGIS macOS Installer Version 3.22 (or later). If you have macOS Catalina (10.15) or later, please note the instructions about security rules: "QGIS is not yet notarized as required by macOS Catalina (10.15) security rules. On first launch, please right-click on the QGIS app icon, hold down the Option key, then choose Open."
Downloads for older versions of the Mac operatating system are available from the web page below. Choose the most recent version that will run on your operating system (e.g., 3.16.1-1 for OS X 10.12 Sierra).
Older versions of QGIS are available for earlier versions of Windows and macOS. However, finding the right one is a little more involved. Send me a note with the version number of your operating system and I'll get back to you with more information.