What does the french word casser mean?

It is a deep sided dish, usually glass or ceramic used to bake foods. It might or might not have a cover.
Ramekins are a particular type of casserole made of white porcelain, with a round flat bottom and vertical sides, often used in making souffles, custards and other individually portioned recipes.

It is a mixture of foods that are baked together in a casserole dish. Often a grain product (rice, pasta, barley...etc.) mixed with vegetables and/or meats combined with a creamy soup or sauce. The foods are usually precooked before mixing and baking them.
Tetrazzini is a particular casserole recipe using egg noodles, cream sauce, mushrooms, and a shredded meat (often poultry or fish), and topped with bread crumbs or crumbled fried onions.