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'Floating' is a technique used in acoustic playing, though there's no reason why classical and electric players shouldn't use it too. Bluegrass players tend to use it a lot.
Basically, it means holding down a note, then playing an open string, and letting the notes ring together.
The first example here is all played on one string - F, E, F, E, and so on. It sounds as you'd expect it to.
The second example is the same notes with the F played at the 6th fret on the B string. The E is played open, and you can hear how the two ring together.
This technique is usually played with a scalar sequence, as on the next page.