How do you spell the word froot cake?

The correct spelling is fruit cake.
This is a cake which can contain various fruits, depending on the recipe. Though it commonly has raisins, sultanas and cherries.

In some countries such as the UK, the term "fruit cake" is also commonly applied to someone as an idiom to describe them as crazy. For example "john's a bit of a fruit cake isn't he?". In the US they tend to use the term "fruit loop" instead.