This is the "Show info for symbol under mouse" feature, which is configured in the Context Tagging section for the language you are using. You can turn this off by unchecking that checkbox. Except for a few minor exceptions, you may find that this is a wildly useful feature, giving you a quick way to preview the definition of symbols.
You can get similar information using the Preview Tool Window, which shows the definition for any symbol at the editing cursor position.