An automatic thought is something you tend to think without thinking about it, as intellectually garbled as that may sound. This might be something like "My OCD is never going to get better." or "There I go doing something stupid again." My homework asks me to replace some of my personal automatic thoughts with new ones. For example, if I thought that my OCD is never going to get better, I would practice thinking something like "I'm doing my best to get better." or "People with OCD get better everyday. I can too."
In order to facilitate this change of automatic thoughts, I was asked to write a few down and look to them several times a day to remind myself. Other than losing the piece of paper I wrote them on multiple times, it seems to be going all right. I have had some opportunities to catch myself in what I hope will soon be my old way of thinking. I'll let you know how it goes.