Sarah,
A follie scan shows follicular growth. As you know the ovum are encased in the follcle. When the follicle is mature the lutenising hormone surge (LH surge) occurs. This rush causes the follicle to burst open and release that gorgeous egg!

Most women only get one ripe follicle a month on a natural/unmedicated cycle. This u/s tomorrow will see how many if any follies you have and the size in mm's. A follicle is thought to be ripe at about 18mm's but sometimes it can be earlier and often it's later.

The u/s will also show your endometrium. The endometrium gives good clues to a woman's fertility. We want to see a triple stripe pattern in best case and a nice thick lining upwards of 8mm's. This shows that your womb is developing well to support a little embie in there!

I hope that helps my love. I am visualising a gorgeous endometrium and some nice follicular activity!