The term "fixed" is a reply that includes a quote of the original post that's been "corrected", in a way that partially or completely changes its meaning, followed by fixed, or fixed that for you, on the next line.
For example, a post saying "FF7 is the best game ever" might be replaced with:
- >"FF7 is the most overrated game ever"
Sometimes the "fixed" is taken to an extreme level where the fixed statement has absolutely no similarity to the original statement. The same statement as above could turn into:
- >"I love to suck meaty cocks in the morning"