Support Loops

Support Loops are a fundamental part of polygonal modeling.

We can see what it does more clearly by adding a Subdiv modifier to the object.
Add a Loop Cut with the shortcut Ctrl+R. Left-click to add the Loop Cut, then move it to where you want. Left-click again to confirm the position.

The closer the loop cut gets to the edge, the sharper that edge will be.

To change the position of the Loop Cut again, select it by double-clicking on it, then press G twice to move it. Left-click to confirm the new position.

There are other, more non-destructive ways to create sharp edges like Creasing or the Bevel modifier.
The benefit of Support Loops is that the model will look the same if you export it to other software.

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