When someone has a bone fusion of any bones, the bones are literally fused together so they cannot bend or move. The surgery, called arthrodesis is done when the cartilage, the tissue in the joints that help them move smoothly, is severely damaged.
To do a fusion, the surgeon uses pins or screws to fasten the bones together. The effect, after surgery, is the toe can’t bend as it used to. It should, however, reduce the pain, making walking easier.
Your mother may have to be careful choosing the style of shoe she wants to wear. Because of the inability for the toe to bend, styles like high heels won’t be possible. She may also have to adapt her walking gait a bit to accomodate the fact that the toe doesn’t bend with the step.