All put right (or left) hands into the centre and dance a complete turn around the centre. This usually takes 8 bars, although timing can vary (note the diagram below is for an 8 bar right hand turn). Commonly performed with two couples or three people (half the dancing couple plus the whole of another couple -- this is also known as teapots).
Half right hands across is also common, which takes you half way round the group of dancers to the diagonally opposite position. This is often followed by left hands across, taking you back to where you started.
Not to be confused with rights and lefts.
Note that right hands across with two people is simply a right hand turn/