This is just defining the edge of the pattern they are discussion. They are saying 'if you see this pattern with any surrounding cells at any point T, then the

This is just defining the edge of the pattern they are discussion. They are saying 'if you see this pattern with any surrounding cells at any point T, then the given cells must also match at time T-1'. If you look at the pattern, you'll see that some of the cells on the edge do not have 2 or 3 neighbours, so if you place this pattern in isolation in a field of empty cells, it won't survive, but there will be multiple possible ways to make this pattern survive (so a choice of cells to make the edge cells add to 2 or 3)