How is this stalemate? My friend played this game as white and wants to know how this is a stalemate. I do not know much about chess and both of us would like to know more about this rule. 

k7/2Q5/8/P2P4/1R6/8/2P3PP/6K1 w - - 0 1

1.Rb6 1/2-1/2
Stalemate is defined as the player's whose turn it is to not be in check, &amp; cannot make a legal move (black's only piece, the king, can only move to squares where it will be in check, which is illegal). 

By playing Rb6 (or possibly Rxb6), your friend walked into that exact situation. Essentially, she blocked off all the squares the King could move to, which is stalemate (&amp; the result is a draw). It might have been a mistake, or it could have been the best move given the situation.

Additional discussion beyond the original question

If there was a black queen on b6, your friend capturing (Rxb6) was the best move, because they would have to get out of check. They can capture with the queen, the rook, or the pawn, but all three lead to stalemate (a draw is all they can hope for here). If they try to move the king out of check, black takes their queen and has a tiny advantage going into the endgame.

Interestingly, if there was a black bishop on b6 threatening the king, the best move would be to move the king and give up the queen. A rook plus several pawns will eventually overpower a lone bishop in the endgame.

If your friend captured anything other than a queen or bishop on b6, that was a blunder that forced stalemate from a winning position. 

If there was no black piece on b6, your friend also had the game basically won. They could have finished it with:

1.Rb8# (Rook moves to the dark square up and to the left of the Queen, and checkmate) 

Another option: 

1.Qb8# (Queen moves to the same square, and checkmate) among other moves. 

The last two cases are perfect examples of how chess can punish or reward you in one move; you can be totally winning and then end up with what is essentially a tie because you messed up.