Many things in Rubygame require you to manipulate rectangles (or "rects"). For example, a rect is used to manage the position and dimensions of a sprite, used in collision detection and can be used to represent any rectangular area in your game. These rects are simple classes with X and Y coordinates, as well as width and height.
Rubygame provides the Rect class to manipulate rects. You can either manipulate the x, y, w and h attributes manually, or use one of the many helper methods Rubygame has thoughtfully provided for you. Since manipulating and positioning rects is such a common thing, Rubygame has provided quite a few helpers for you. Here are a few that will be most helpful.
bottom, left, right and top - Position the rect so that the bottom, left, right or top are at this X or Y position. This can be useful if the player has gone off the screen to the left, as you can assign the player's rect's left to the left side of the screen.
center - By default, rects are most of the time referred to by their top-left coodinates. This is not terribly useful in most cases, so Rubygame provides the center method. Using the center method, you can move the center of the rect to the X and Y coordinates you give it. This is a much handier method of moving things around than moving the top left corner.