Basically, whenever you have two different atoms, such as here HF, we're going to have an equal distribution of charge. Electrons are going to be closer to the larger atom because this f, for example, has nine protons, while this h has only one proton. So our nucleus will pull these electrons stronger than the h. So our electrons will be closer this way. Now, another way of representing this unequal distribution is by using this arrow. So we draw an arrow towards where our electrons are being pulled and our electrons are being pulled towards the larger nucleus. So towards the f.
And we draw kind of a plus sign on the end where there's a partial positive charge.