Usually with tab, a 'x' means mute the note, so it sounds like a dull thud rather than a proper sounding note - like you may do lots of the time as a beginner! This tab is being helpful in that it shows the note values (lengths) too. That's why the vertical line is there, it's one beat long.Two lines joined together are half a beat each, and as Kirk says, with a '3' under or over, 3 will be played in the time of two - triplets.
Having said that, it may be that the 'x' is what you actually play whilst holding down the chord as shown. Incidentally, the chord 'F7' in the 1st window should be called Fmaj7, slightly different, as the top string shows an open E note, not found in 'F7'.
You will learn over time that a heck of a lot of tab available on the internet is inaccurate, to say the least. Trust your ears more than tab!