Two pair is a poker hand that ranks above a pair but less than three of a kind. A two pair hand consists of two sets of two equally ranked cards along with a fifth card - called a "kicker".
To determine the rank of two seperate hands ranked as "two pair" you first compare the highest pair in each hand. If they are the same then the lower of the two pair in each hand is compared. If this is also the same then rank of the remaining "kicker" card determines higher hand. If the kicker is the same then the two hands are tied and the pot is split between the two players.
The most well known example of two pair is the Dead Man's Hand which consists of two aces and two eights.