Meaning: Take a remedial action to prevent a complete breakdown. Like when water begins to breach a dam, it is easier to repair the breach than to repair the dam.
Similar proverbs:
An ounce of prevention is worth a pound of cure.
A stitch in time saves nine.
Proverbs 17:14
The beginning of strife is as when one letteth out water: therefore leave off contention, before it be meddled with.
Psalm 106:23
Therefore he said that he would destroy them, had not Moses his chosen stood before him in the breach, to turn away his wrath, lest he should destroy them.
Isaiah 58:12
And they that shall be of thee shall build the old waste places: thou shalt raise up the foundations of many generations; and thou shalt be called, The repairer of the breach, The restorer of paths to dwell in.