A Singles Chain consists of an unbroken series of links, where a link is a line joining two candidates within a unit when there are exactly two candidates of that value within the unit. In this chain, the candidate value is 6. For each link of the chain, one of the linked candidates will be a solution, while the other will be a non-solution. All of the solution candidates have the same color (either green or red) while all of the non-solution candidates have the other color.

The cells highlighted in blue in row I draw your attention to the fact that there are two red candidate 6s in the same row. Obviously, they can't both be solutions, so they must both be non-solutions. In fact all of the red 6s in the puzzle must be non-solutions and may be removed. Naturally it follows also that all of the green 6s in the puzzle must be solutions for the cells they occupy.

Simply put, Rule 2 states that same colored candidates within a unit must be non-solutions.