When you send a packet, it gets stamped with the number 127, and each router that it passes through decreases it by one. When it reaches zero, the Time To Live has expired and the packet is no longer forwarded anywhere. This feature was implemented because people thought internet packets might get stuck in loops, so if they don't reach their destination in 128 hops, they are killed. I've seen TTL expired errors when there's a routing issue, like when two routers keep sending the packets back and before between each other until the TTL expires. Not sure what would be causing it for you.