Let me try to be as simple as possible.
The two slits prevent the electron from hitting the screen unless it passes thru one of the two slits. How? The metal sheet interacts with the electron thru the ElectroMagnetic (EM) force. In other words, the EM force is interacting with the electron, limiting the possible positions it can have at the metal sheet. When an electron hits the screen, we hence know that the electron had a limited possible positions at the metal sheet area.

It could have passed thru any point in the two slits. That represents the so-called "uncertainty" in its position.

As we make the slits thinner and narrower, we are using the EM force to try to lower the possible spread in the positions that the electron can have. This force or interaction also causes one other thing: it increases the possible momenta that the electron can have. This is called the uncertainty in the electron's momentum.

Notice that it is the EM force that causes an electron to increase the spread in its possible momentum as you decrease its possible positions. In other words, the Heisenberg Uncertainty Principle is derivable from Quantum Field Interactions.