If you remember the 5 sector circular flow from year 11, you can use that to visualise AD. Think of demand as wanting to buy something from a firm, and therefore aggregate demand as the sum of everyone who wants to buy things (goods and services) from firms.
So, all the injections into the circular flow go to firms, so you can think of Gov spending (G), Investment (I), net exports (X - M) as all the money flowing to firms demanding goods. Also income spent my households goes to firms, so include Y.
That's how you get AD = Y + G + I + X - M.
Those respective parts are the elements of AD.