Those are harmonics: you very lightly touch the open string at the twelfth fret while playing the string and you should hear a bell-like sound an octave above the open string. The same for <7> except you should touch the string at the 7th fret and you should hear a note a twelfth above the open string.
There are plenty of tutorials on the net, for example here.
They are open string harmonics, found at various places along the string, each of which will divide the open string into equal parts - 1/2,(12th fret), 1/3,(7th fret), 1/4, (5th fret) and so on. Gentle touching over the fretwire will enable them to be plucked and sound.
Look at the standard music portrayed above the tab - instead of 'round' dots, the harmonics are signified by diamond shaped dots.
It's also common to play harmonics one handed - touch the string with thumb or finger, and pluck with finger or thumb. And the 7th fret and 5th fret harmonic nodes are also found at the body end of the string - over the 19th and 24th fret respectively.