When you take in more sodium, there is a higher concentration of solutes in the vascular space (in your vessels). Remember, water follows the solutes, so when you have an excess of solutes in the vascular space the water will leave your cells (causing them to shrink). When your cells get smaller, your thirst sensation is triggered, making you thirsty.
Thirst is triggered by osmoreceptors in the hypothalmus. As their name suggests they sense changes in the osmotic pressure of blood. These cells expand (take in H2O) when the blood is hypotonic and shrink (loose water) when the blood is hypertonic. This shrinking triggers the cells to send an afferent signal to the brain triggering thirst. So when you eat salty foods you increase the amount of sodium in the blood making it slightly hypertonic and making you thirsty.
Sodium is something your body needs when you’re trying to rehydrate, either during or after exercise. That’s why sports drinks are often rich in sodium — one of the “electrolytes” your body loses during exercise. Drinks and snacks with sodium also can trigger thirst and help you retain fluids. But too much salt can raise your blood pressure and worsen heart conditions in some people.