Quick answer
A quarrel starts as a trickle through a crack and ends as a flood. The counsel is to plug it early, because later there is nothing to plug.
Read both lines
The image is a breach in an earthen dam or an irrigation channel, and anyone who has seen water find a gap knows the sequence: a seep, then a channel, then the whole wall. What makes the picture exact is that the volume is not the problem at the start and cannot be managed at the end. The second line's counsel follows directly and is unusually practical: the moment to act is before, which means the skill being recommended is recognizing a beginning. Most people only recognize a flood.
Related: Proverbs 17:1 · Proverbs 20:3 · Proverbs 26:20–21
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.