In guitar tabs, an “x” is used to indicate a string that should be muted or not played.
For instance, if a tab has the numbers “3” on the top line and “x” on the bottom line, it means you should play the 3rd fret of the top string and not play the bottom string. The “x” is used to indicate that the string should be muted or not played.
It is important to note that the x notation is used in tabs and not in standard notation.
In standard notation, it is represented with a line through the note or a “0” (zero) or “silent” written above or below the note.