Look at the cell D2 which is highlighted in green. It contains the candidate numbers 2 and 9, so one of these must be the solution for that cell. If 2 becomes the solution in the green cell, 1 will become the solution in the red cell at F3, and if 9 becomes the solution in the green cell, 1 will become the solution in the red cell at G2. In other words, one of the red cells must have a solution of 1. The blue cell can 'see' both the red cells, and so cannot have a solution of 1. The 1 candidate in this cell is highlighted with magenta to indicate that it may be removed.