Markers are movable indicators that you can create wherever you like on the
time scale - just double-click on it and it'll create one for you where your
-mouse cursor is at the time, or use the context menu when right-clicking.
+mouse cursor is at the time, or use the context menu when right-clicking on
+the ruler or a signal trace.
You can click on its label and you'll have the option to change its name, or
drag it to reposition it.