I'm not sure if you're being serious... but... cross chaining is basically where your chainline isn't straight. So, example, you're in the big ring and your biggest cassette, or vice versa. A la:
I was being serious but I'm not sure why I bothered asking when I went and Googled it anyway. Prior to asking, I did realise that's probably what it meant but didn't think it would actually be that simple. Oh dear, dear
I'm not sure if you're being serious... but... cross chaining is basically where your chainline isn't straight. So, example, you're in the big ring and your biggest cassette, or vice versa. A la: