X is a mute. You rest your hand (or finger) on the string it mentions (in this case A) and pluck. It'll sound flat and dull but it makes a percussive sound that you'll find important in a lot of rock stuff.

As for the / (like 3/5 or 5/7 etc.) this is a slide and if the tab said
----------3/5-------- then you would play the 3rd fret, then while the note's still ringing, slide that finger up to the 5th fret without picking again.

If you need any more help ask :)