How long to use a saved copy
A café might use a printed menu for an hour before checking for changes. A cache works the same way. With a ten-minute TTL, it can use saved information for ten minutes before checking whether a newer copy is available.
DNS answers also have TTLs. If a website’s address changes, devices may use a saved old answer until its TTL ends. A shorter TTL helps the change appear sooner but means checking the address more often.
A different meaning for packets
For an IP packet, TTL counts network steps, called hops. The packet is dropped when the count runs out, stopping it from travelling in circles.