It means players from Chelsea and Reading are eligible for double points since they are playing twice in that week. Suppose you pick Torres and Lampard (who are both expensive) in your team and they score a goal in both matches. This means you get close to 30 points from them this week. If it was a regular week and they only played one match, you will only get like 15 or so points.
There's also a downside to this. Torres and Lampard can easily flop (benched/no goals or assists) and end up getting like 5 points altogether from two matches. That's not a good investment and you're better off picking a guy who only plays one match a week but has a favorable fixture. For example, RvP vs West Ham.
Personally, I don't recommend stacking up on Chelsea and Reading players this week. The Chelsea guys cost a lot and some of their new transfers might be benched (Oscar/Marin) and the others need time to adapt to the English game (Hazard/Azipilicueta).