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 numbers (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>.