Complex sentence: Development is sometimes followed by immersion in a mildly acidic stop bath or rinse to halt the development action, and then by a combined bleach-fix which both dissolves the silver generated by development and removes undeveloped silver halide.
Split into simpler sentences: Development is sometimes followed by immersion in a mildly acidic stop bath to halt the development action. A combined bleach-fix dissolves the silver generated by development and removes undeveloped silver halide.