To 'hold something against someone' means to have a negative attitude towards someone based on how they behaved in the past. When you hold something against someone it is the same as 'holding a grudge' meaning you find it hard to forgive past wrongdoings and move on.

1. Person A: "I can't stand Andrea, especially after the way she behaved at the party last weekend." Person B: "You can't hold that against her - she'd just had too much to drink."
2. I don't believe in holding things against people. iI someone apologises to me I am willing to let things go.

1. Luke did leave the company in the lurch before, but he's proven his reliability since then. It's not fair to keep holding that against him.
2. Tom's mistake cost us a lucrative contract, but it could have happened to anyone. I won't hold it against him.