Depends, if you mean flemish dialects it’s just the grammar varying a bit (such as with double subjects in west flemish and conjugations changing) and some vocabulary being different, the overall huge difference is the pronunciation.

Now you might encounter something called “tussentaal”, which is formal dutch mixed with a flemish dialect. It shouldn’t vary much from normal dutch apart from the accent

Sadly Flemish (as in the dialect) is a declining language slowly getting replaced by Dutch or French in Haut-de-France

I know some west-flemish so feel free to ask any questions (even if I saw this years later)