Originally Posted by Glenn Gould Fan

I'm an early intermediate player, so forgive me if this is a simple question. This is also my first post on this site.

I sometimes see a4 played by white but I do not know what kind of plan this move suggests. What does a4 do? Is a4 an aggressive move, or a passive move?

If it helps, a4 is a viable option in the following early position:



What does an a4 move in this position do, as opposed to the more standard-looking moves of d3, or O-O?