Disney will usually send you an email alerting that you can do online check in. You can do it starting 10 days before your check in date. You just click a couple of things, put a credit card on file if you wish, and it supposedly streamlines the check in process when you get there. There will be a separate, specially marked line for people who have done online check in.
I almost always use it and it seems to save time since they have all your information right there and your info packet ready. The line, if there is one, is usually shorter than the regular check in line.
Some people don't like it, some do....it's up to you to try it or not. I think it does save a few minutes so I use it.