Read the Sudoku help page for the rules, and a summary of the techniques required to solve BrainBashers Sudoku.
Forced Moves is one of the basic techniques people use when solving Sudoku.
When there is no choice (because the other numbers already appear in the row, column and block), a square is forced to be a particular number.
R5C2 can only be a <2>, as no other possibilities are allowed.
R1C4 can only be a <7>.