I like AdGuard's description of TTL:
Time-to-live (TTL) sets the time period (in seconds) for a client device to cache the response to a DNS request and retrieve it from its cache without re-requesting the DNS server. If the TTL value is high, recently unblocked requests may still look blocked for a while. If TTL is 0, the device does not cache responses.
How to clear cache https://adguard-dns.io/kb/public-dns/solving-problems/how-to-flush-dns-cache/
I like AdGuard's description of TTL: