Red Hat holds patents but promises not to enforce them. This means anyone can use the IP without worrying about being sued for infringement. No one else can claim they own the IP because Red Hat already owns it. They also have a clause saying that if Red Hat sells the IP, the buyer needs to uphold the same promise.

The "combined" part seems to be saying you can also develop a system that's partly proprietary, as long as the proprietary parts are simple and not using Red Hat's patents.

If they sell a patent, the buyer must agree to the same promise.

The promise won't protect you if you infringe someone else's IP, even if it's related to one of Red Hat's patents.

If I read this right, Red Hat promises to not sue anyone for using/transferring FOSS whether or not such an action infringes on Red Hat's patents - they will only use patents to defend against litigation. (IANAL.)