there's another thread on this too, Ryn explained it really well.
Everyone has 2 genes for eye colour. Brown is dominant, blue/green are recessive
So you can have brown eyes and have either 2 genes for brown eyes, or 1 for brown and one for blue.

In my example, I have blue eyes, which since its recessive, and no one else in my family has green, all are blue, means I have 2 genes for blue eyes.

DH has brown eyes, as did his dad. His mum has blue eyes, and BILs are green/hazel

So DH has 1 brown gene (cos his eyes are brown) and one blue (cos his mums are blue and his brothers are green)

I'm not sure quite how green works, but DS has brown. Our kids have a 50% chance of brown eyes, 32.5% chance blue and 17.5% chance green