Bleeding brake fluid is the procedure involved in removing air bubbles in the brake lines. The presence of these air bubbles reduces the pressure within the brake lines, and must be removed. The process of bleeding brake fluid involves completely changing the current fluid. Changing brake fluid is a process to remove any moisture that has built up over a long period of time. This moisture will affect the efficiency of the fluid and must be removed. This involves completely changing the fluid. Bleeding brake fluid and changing brake fluid involve the same process and require a complete change in fluid.
For more info, see How Often Should You Change Brake Fluid