Welcome aboard,

Lot of players starting or restarting after a long time away in that age range so your in in the right place.

And yes the "x" would signify not playing or muting the string.

Also, I just noticed on the home screen there is a great piece on sore fingers that guitartricks has addressed because it's a common problem with beginners. I'd encourage you to give it a read to help out with that.

https://www.guitartricks.com/blog/

Good Luck and no it's not easy but it's fun and this site will get you through it. Have patience, keep your expectations in check and progress through the lessons at your own pace. Most of all enjoy.

Moe