When it comes to problem-solving, often our brain stands in the way to figure out the best solution. Our memory stores typical uses of objects and previous solutions to a problem. These work as an inventory when we are trying to find the way out from a problem situation - such a relief! You have a bunch of things to choose from. This is great until you want something unique and more creative. THEN you need to "let go" your earlier patterns and memories and start with a blank page, which is not that easy as we think. Functional fixedness means that - unless we train our brain to be more flexible - we tend to stick to the original functions and uses of objects, we can hardly imagine solutions involving using them in a different way. (When you see small children, they happily turn a bowl into a hat or start playing with spoons at the table - they don't have functional fixedness - yet.)
See this short video made by the Brain Games crew at National Geographic Channel and see if you could solve their test: