How does heat affect pressure?
Well, pressre is basically collisions and the hotter you are the faster you go and the harder you collide (if you are a gas molecule I mean!).
Suppose you have a certain amount of gas in a container then pressure will be given by the continuous collisions of the molecules of the gas (moving around inside the volume of the container) against the walls of the container (you can see this if you substitute one of the walls with a movable piston; it will move under the impacts of the molecules).
Now, if you give heat to the gas basically you transfer energy to it (and the gas becomes hotter, meaning that the molecules of the gas will "use" this energy to...go faster!!!).
The molecules of the gas increase their kinetic energy basically bouncing harder against the container walls. The pressure as result of these harder impacts will increase (as in a steam pot).