(half)abSinC
Lets do some computational thinking and decompose the formula. (Decomposition is the process of breaking down large problems into smaller problems that can be solved to solve the larger problem)
SinC - this is the angle between the two sides which are taking to find out the area. (you will understand this later when you read on). By doing inverse sin, you will be able to find out the angle (again explained more clearly further down)
"a" and "b" are two sides, which you are taking the length. These two side must meet each other (so "a" must meet "b") and you must know the angle between them in order to take these lengths.
(half) - area of triangle - its standard
For GCSE, you don't need to know how the formula came about
To find angle in a question, I assume they gave you the area(or you might have to work it out somehow) and they have given you the length (or you might have to work it out). Don't worry, only one type of information might be missing, so you only need to find out one thing most of the times.
If you rearrange the formula of (half)abSinC to make C (the angle) the subject, you would get C = (Area*2)/(ab), which you can use to work out angle