Let's suppose you want to create a game with a camera that has its position restricted within a triangle. Remember that a triangle is the space enclosed by three lines formed by drawing a line between any three points. This means that the camera cannot leave this space!
This is probably best visualized by taking out a sheet of paper, drawing a triangle, and shading in the triangle. If this piece of paper represents your game, your camera can only be within the shaded region.
Of course, this is easy if your camera doesn't have to move; you just have to place it somewhere within the triangle!
So, we want our camera to be able to move so that it is as close to your character's head as possible and is pointed towards the character's head so you always have a good view of where the character is.
To solve this problem, you might want to look at some wiki pages! Here are some that may be helpful: