What do the Terminal menu item icons under Window mean?
A diamond (♦︎) means the window is minimized (as user3439894 said in their answer). But a dot (•) means the window is "dirty" — that is, it can't be closed without potentially losing something. An app will typically ask for confirmation before closing a dirty window.

These conventions apply across macOS's UI, not just Terminal.

For Terminal, dirty means that there's a program running in the shell, other than the ones listed as safe in Preferences > Profiles pane > Shell tab > Ask before closing.