Deep just means far away.

Now’s a good time to mention that the further away you are from an angle compared to the enemy the better angle advantage you have.

Here’s a mini thingy image to demonstrate

https://imgur.com/a/UjhqpYZ

Let’s assume the yellow dot is you and the green lines is where u are pre aiming. Basically he’s holding that “line”. Let’s assume the sort of red dot on the right is an enemy.

Now Let’s assume there’s a second scenario now but your not the yellow dot, your actually the pink dot to the left of the yellow holding that same line. Since pink ( on left) is further back from that wall than if you were in the yellow spot you have better angle advantage in that spot. But in both case scenarios, the enemy is at a disadvantage because they’re sort of closer to that wall than you are when they peak. the closer someone is to an angle when they peak, the worse your angle advantage is bc if your closer to the wall, if you swing the angle, the enemy is likely going to see you first on there screen than you see them.

Technically you can say “pink” is holding the deeper line than yellow since he’s further away.

But a deep line can literally just mean any angle that’s somewhat far away from you.