The "Offline" status merely means that the SEPM has not heard from the client within its configured heartbeat interval. The reasons behind it are numerous (as earlier posts have indicated) and could range from a simple network blip, to the client having been vapourised in an alien attack (maybe a little unlikely ), and everything in between (for example: client is off, client is disconnected from the network, a firewall is blocking comms, client is trying to connect to SEPM via a proxy, client cannot resolve SEPM name, etc, etc, etc).
The "Offline" status is only really cause for concern if you think those client should be online. In which case, it's time to get a hold of one of those clients and do some investigation (Ashish's links should help here, and "Thumbs Up").