"Fence matching" is an optional feature that calls your attention to the opening bracket when you enter a close bracket, or vice-versa.
The visual appearance of fence matching is a blue circle, which quickly animates outward from the bracket that matches the one you just typed.
The fence matching animation will also appear if you move your text cursor over an opening or closing bracket using the arrow keys.
To turn fence matching on or off:
Coda can optionally close brackets automatically as you use them. For instance, if you type an open parenthesis, the closed parenthesis will automatically appear after your cursor.
Should you type the closed parenthesis anyway, it will overwrite the automatically inserted one.
To turn bracket completion on or off: