The Design Brief is a short statement of what you are going to make, why you are going to make it, and for whom you are making it for.
It should be an open ended statement.
For example; A clock project.
A good Design Brief might be something like this...
"Teenagers have very busy lives. School, family and social activities mean that being punctual is very important.
I will design and make a wall clock that will be appealing for teenage girls. It will be a modern design and will be suitable for a teenage girl's bedroom. Because the design will appeal to teenage girls, then they will be more likely to look at it, read the time and therefore not be late."