Essentially it involves 'jumping on the tails' of a good joke or comment that has just been told by someone else by either repeating it or changing it slightly to try and obtain or claim the same laughter as the initial joke or comment.
This is known as 'tailsing'
It is technique often used by lazy or poor comedy hosts - for instance Dara O'Briain on BBC's 'Mock the Week' makes a living from using this technique as he does not appear to have any jokes of his own.
It is also used by people who think they are funny but lack the timing or genius of true comedy greats. They will wait for someone to say something funny, repeat it or change it slightly and hope they are remembered for the gag as they said it last.
Reaction: Laughter from the group
Unfunny person (tailsing) " Yes you really are an idiot"
Reaction: Laughter continues from the group but this is left over from the initial gag - but the unfunny person thinks it is for him