The Reti Opening is a chess opening characterized by the move 1. Nf3, which is named after the Czech-Austrian chess player Richard Réti. It is considered a flexible and versatile opening, as it allows for a wide range of follow-up moves and can lead to a variety of different positions.

One of the main advantages of the Reti Opening is that it allows White to control the center without committing to a particular pawn structure. This can make it difficult for Black to choose an appropriate response, as the pawn structure can change depending on White's follow-up moves.

The Reti Opening is also a popular choice among top-level players, including Garry Kasparov, Bobby Fischer, and Hikaru Nakamura. It is also considered a good choice for players who want to avoid the more theoretical openings, such as the Sicilian Defense or the Ruy Lopez.

The main responses are d5 based setups, and e5 based setups. Learn how to deal with them in the according PGN file, and youtube video provided. 

In summary, the Reti Opening is a flexible and versatile opening that allows for a wide range of follow-up moves. It allows for a quick development of the knight and bishop, and can put pressure on Black's pawns and pieces. It is a good choice for players who want to avoid the more theoretical openings, and is also a popular choice among top-level players.