Crop means to cut something down – like taking a pair of scissors to a picture but doing it electronically – WordPress.COM has a reputation for not scaling pictures well at times for things like headers etc.
If you are uploading a header picture you want to fix it with some type of photo editing program before uploading it – the picture should be the exact size of the header.
Photo Shop is what many use – there are a lot of them – I use Gimp that is like Photo Shop but free. bit to get used to but for simple things like crop and scale it is not too bad. Look at cnet.com for the download if you want.
Better yet – find a friend that is good at graphics and buy them a drink and get them to fix your picture for you. :)
A header is a picture at the top of your blog – the site linked to your name does not have one. I also don’t see any pictures on your blog – are you asking about a different site? If so we need a link to the site you are asking about.