The phrase "everything but the kitchen sink" means too much stuff

, too many things (most of which are unnecessary), everything you can imagine. In this phrase, "but" has the meaning of "except": you can find absolutely everything here, except the kitchen sink.
I guess we use "kitchen sink" here as a symbol of something large, bulky and unnecessary in most situations.
Here are some examples:

"Your room is too cluttered, you've got everything but the kitchen sink in here!"
"Why did you bring so many things for a short trip? You've packed everything but the kitchen sink!"
"Your essay is too long, you've written about everything but the kitchen sink!"
"Your friend really likes to eat! When he visited us for dinner, he ate everything but the kitchen sink!"