It can be used figuratively to refer to:
* Strengthening a bond or relationship: This is the most common usage, implying making a relationship stronger and more secure.
* Making something more secure: This could be used in contexts like securing a deal, strengthening a plan, or even tightening a physical knot.
The literal translation of the words is "tied knot" but the phrase implies a process of making the knot stronger, hence the phrase "tightening the knot."