HLTL stands for "Hop Time To Live" which is the number of routers a packet can pass through before it is discarded. Each time the packet passes a router the HLTL is decreased by 1 and if it reaches 0, the packet is dropped.
TTL stands for "Time To Live" and it is similar to HLTL, but it measures the time in seconds that a packet can exist on the network before it is discarded. The TTL is decreased by the amount of time the packet spends on each hop, and if it reaches 0, the packet is dropped.
In summary, HLTL is the maximum number of routers a packet can pass through before being discarded, while TTL is the maximum amount of time a packet can exist on the network before being discarded.