What is the role of a fixer in black and white photography?

Films and photo papers contain light-sensitive grains called silver halides. When you develop, the silver halide grains that were struck by light are chemically reduced to black metallic silver. The remaining silver halide grains have to be removed from the material or they'll get dark and ruin your picture. Fixer dissolves the silver halide grains and the water wash which follows removes them so your picture will be permanent.