A TXT record is a DNS entry that holds arbitrary text rather than an address. Hosts commonly use one to prove you actually control a domain before letting you point it at their service, and email systems use them for sender verification.
Not every host needs one. If the domain already points at the service's own address (an A or CNAME record it can look up), that lookup itself can prove ownership, so a separate TXT record is not always required.
For when a TXT record is actually needed and when it is not, see connect a domain you already own.