Work = The Scalar Dot Product between Force and Displacement. If you apply a force on an object and it covers a displacement IN THE DIRECTION OF THE FORCE you have supplied ENERGY to, or done WORK on, that object.
Scalar Dot Product?
A product is obviously a result of multiplying 2 numbers. A scalar is a quantity with NO DIRECTION. So basically Work is found by multiplying the Force times the displacement and result is ENERGY, which has no direction associated with it.
A dot product is basically a CONSTRAINT on the formula. In this case it means that F and x MUST be parallel. To ensure that they are parallel we add the cosine on the end.