You are disappointed when something you expected or wanted doesn't happen. People might get disappointed that the last slice of cake was taken before they got a chance to have some.
A focal point is where rays of light converge into a single point. The glass inside a magnifying glass is curved so that it bends all the light that goes through it towards a central point. If you move a magnifying glass up and down in the sun, you will find that at a particular distance away the magnifying glass makes a pinpoint of light that is hot enough to set something on fire. This is called the focal point.