Explain the French drain

[Xfitter71]

And how it works. And, is it legal to use it for washing machine waste water.

[pushkins]

A French drain is a long trench with a layer of drainage rock then a perforated (usually 4") pipe laid on top of the drainage rock bed then completely covered with drainage rock. Usually the rock will go all the way up to about 4" from the ground surface with soil and grass above. Often I use fabric weed mat to protect the gravel bed from soil infiltration.
Mostly they are used for water redirection like at the back of a house to guide the water around the house, or to dry out a crawlspace by wrapping it all the way around the home and directing the end downhill or to a culvert etc...
Washing machine water also known as "grey water" can be sent into a recycling system (garden use, lawn use etc...), it can be area sensitive so you'd be advised to ask your local building department. It cannot be redirected to culverts, storm water areas etc...
You would need a size appropriate French drain depending on your washing machine size, it wouldn't be really a "drain" as much as it would be a leach field for the machine water.