TTL, short for Time To Live, is a value attached to every DNS record that tells resolvers how many seconds they may cache the answer before discarding it and querying the authoritative server again. It is the throttle that balances DNS performance against how quickly changes take effect.
A high TTL, such as 86400 seconds (one day), means resolvers cache the record for a long time. This reduces query load and speeds up lookups, but it also means any change you make can take up to that full duration to be seen everywhere, because cached copies keep serving the old value until they expire.
A low TTL, such as 300 seconds (five minutes), makes changes propagate quickly at the cost of more frequent queries. The common best practice before a planned migration is to lower the TTL well in advance, make the change, confirm it, and then raise the TTL back up for efficiency.
For email operations, TTL matters whenever you edit MX, SPF, DKIM, or DMARC records. Understanding TTL prevents the frustration of editing a record correctly and wondering why mail still behaves as before - old cached values are simply still in effect until the TTL expires.