Someone's bread and butter means something that gives them their regular income.
You can't ask your neighbour to fix your car for free. He's a mechanic and fixing cars is his bread and butter.
Mary was very helpful to her clients. She knew they were her bread and butter.
Don't confuse this expression with to butter up. This phrase means to flatter someone so that they like you and you can get something from them.
Fred was new to the company but he didn't waste time before buttering up the boss.
Bread and butter pudding is a very traditional, simple British recipe. Its basic ingredients are sugar, cinnamon, butter, bread, sultanas, milk and eggs.