Read the Sudoku help page for the rules, and the techniques potentially required to solve BrainBashers Sudoku.
BUG removal is an advanced technique. Its name comes from 'Bivalue Universal Grave'. We are looking for a situation where every remaining square has two digits (bivalue), except one square (polyvalue).
If R1C8 was <18> then we would have a Sudoku in which every remaining square had two values in each. In addition, every row, column and block would all contain each unsolved possibility exactly twice. This is called a Bivalue Universal Grave (BUG) pattern and isn't allowed. The Sudoku would either have no solution at all or have more than one and not be unique. Thus R1C8 cannot be <1> nor <8>. Hence <1> & <8> can be removed from R1C8 leaving <4>.