The term "TTL" refers to "Time to Live", which is a value in an Internet Protocol (IP) packet header that tells a network router whether or not the packet has been in the network too long, and should be discarded. The TTL is set by the sender of the packet and is decremented by one every time the packet passes through a router.
In IP networking, the TTL is used to prevent a packet from circulating indefinitely in the network in case of a routing loop, where the packet keeps being sent from one router to another without ever reaching its destination. When the TTL value of a packet reaches zero, the router discards the packet and sends an ICMP (Internet Control Message Protocol) "Time Exceeded" message back to the sender, indicating that the packet was not delivered because it exceeded the maximum time it was allowed to spend in the network.
The TTL value can also be used to limit the scope of a network broadcast or multicast, or to restrict the number of network hops that a packet can traverse before being discarded.
Here are some links that may be helpful in learning more about TTL:
Wikipedia page on Time to Live (TTL) in computer networking: https://en.wikipedia.org/wiki/Time_to_live
Networking Tutorials: Understanding Time to Live (TTL): https://www.guru99.com/time-to-live-ttl.html
TTL: Understanding the Concept and Its Significance in IP Networks: https://www.cisco.com/c/en/us/support/docs/ip/time-to-live-ttl/13753-understand-ttl.html
TTL and Traceroute: https://www.cloudflare.com/learning/ddos/glossary/ttl-and-traceroute/
Understanding TTL (Time To Live) in IP packets: https://www.reddit.com/r/networking/comments/2ukgjt/understanding_ttl_time_to_live_in_ip_packets/