Original: Marcellina has hired Bartolo as her counsel, since Figaro had once promised to marry her if he should default on a loan she had made to him, and she intends to enforce that promise.
Simplified: Marcellina has hired Bartolo as her adviser, because Figaro had once promised to marry her if he was not able to pay back some money which she had lent to him, and she wants that promise to come true.