It is quite complex, trying to explain without a chart. A Family history program will often identify relationships for you.
Draw this on paper. Start off at the top with 2 brothers, side by side. Their children are 1st cousins to each other. Go down one more generation to the children of those 1st cousins. They are then 2nd cousins to each other because they are on the same generation level. The next generation of children on the same level will be 3rd cousins to each other. "Once removed" is when you have the children of one generation linked with a higher, or lower, generation level, and so it goes on ad infinitum, and gets more complicated!
Brothers John Harry
1st cousins Fred Mary
2nd cousins Joe Gladys
3rd cousins Edward Susan
I don't know if this explanation will help or confuse you!!! The layout on AB won't spread these out as I want.
I have met up with a 3rd cousin - our Great grandfathers were brothers, so we share a common GG grandfather and GG Grandmother