In macOS, some apps have semi-transparent panes and toolbars. If something colorful sits beneath them, these surfaces show a blurred version of the colors.
Sometimes the underlying content below is animated, and this can be distracting to some folks, especially those with vestibular disorders. However, it’s easy to remedy this.
Go to System Preferences > Accessibility > Display.
Click the option to “Reduce transparency.” The transparent areas of the desktop and app windows become gray. Semi-transparent panes and toolbars will become solid (as will the macOS menu bar and its menus).
You can also enhance the clarity of your Mac’s screen by enabling the “Increase contrast” option. This darkens the pane and button borders. I find it a bit much, but, thankfully, I don’t suffer from any vestibular disorders.