statistical techniques

I assume that you are asking in respect of regression analysis (which is not the only statistical technique in the syllabus!).

y is the variable that depends on the other variable (x).

So if, for example, you are finding a relationship between the sales and the amount spent on advertising, it is the level of sales that will depend on the amount of advertising, so sales will be y and advertising will be x.

I do suggest that you watch our free lectures – they are a complete course for Paper F2 and cover everything you need to be able to pass the exam well.