The third rank defense is the most important defense in the king and rook versus king, rook and pawn endgame. A lot of people refer to it as the Philidor position, but this name is also used for some other endgame positions.
There is already a lot of information about this position available on the internet, for example in the Philidor position on Wikipedia and I will limit this post to some additional information which probably helps to really understand this position.

First of all it is important to understand that the weaker side strives to capture the pawn or to exchange the rooks while being in a position that the pawn can be stopped. Therefor it is needed to be familiar with the single pawn endgame and the key squares. However the most important part of this endgame is creating a possibility to start a never ending king hunt by the defending rook which also results in a draw.

The third (or sixth) rank defense can be summarized as:

• The black rook is keeping the white king away from the sixth rank.
• As soon as the pawn moves to the sixth rank the defending rook has to be moved back
• starting a kind of king hunt with distant checks