The browser provides visual feedback when a control receives focus. The default visualization usually matches how focus is rendered on the host platform: On Windows this is a subtle dotted outline, on MacOS a blue glow.
Windows: 
Mac: 
The focus outline is important in that it both directs the users attention, and serves as a cursor for users who navigate using the keyboard (most commonly by pressing Tab or Shift + Tab). Often the focus outline is removed using CSS, making navigation impossible for keyboard users.
