To make a mess, to create chaos, to screw things up.
It's often used in situations where someone is causing trouble, making a mistake, or generally behaving in a way that disrupts things.
Here are some English equivalents:
* To make a mess of things
* To cause a ruckus
* To screw up
* To cause trouble
* To make a big mistake
Remember, "fout la merde" is a very informal and vulgar expression, so it's important to use it with caution and in appropriate contexts.