| This is something that my old instructor called a "rhythm click," though I think the proper name is "ghost note" (?). If I'm wrong on the name, I hope someone will correct me.
Anyway, the x tells you that you should play that string at that time but without without letting a note ring out. It needs to be muted. Usually leaving your finger on the string without pressing down hard enough to fret a note is sufficient to accomplish the muting. Sometimes this will cause harmonics (they will sound like high notes), in which case you need to mute in a different spot or rest more than one finger on the string. This should be pretty simple, but let me know if you are confused at all.
I should also say welcome to Talkbass/congratulations on picking such a wonderful instrument. There is a subforum in the bass guitar forum devoted to technique, and this topic has probably been discussed there before. | 
Sup 
