Its hard to say exactly. In most cases they are either looking for something to eat or drink or they are farming aphids.

If there are some aphid eggs somewhere on the plant they will guard the plant and wait for them to hatch. In time you may see them farming aphids. I've seen it many times in my backyard.

Here is a link that might be useful: Herding Aphids: How 'Farmer' Ants Keep Control Of Their Food

Cool pictures!

Well so far, no aphids. It's pretty hot here right now and I don't usually see aphids this time of year but they must be around in some form somewhere, awaiting more favorable conditions. I don't know if it can be seen clearly in the 2nd and 3rd picture, but the big ants with the large mandibles are biting down on the edge of the leaf near the stem.