For large project, it may be slow to do auto recalculation everytime input is changed. User can use this button to pause the auto update. Click analysis lock to manually recalculate.
You will not see the toolbar button if you upgraded from previous version. Choose View: Toolbars… menu. Go to Button Groups tab. Choose AutoUpdate Group. Drag the Pause button to any existing toolbar location.
Or on Toolbars tab, choose Reinitialize button. But it will initialize all your toolbar and docked window layout, including Project Explorer, Apps, Object Manager, etc.