Sender Policy Framework (SPF) records allow domain owners to publish a list of IP addresses or subnets that are authorized to send email on their behalf. The goal is to reduce the amount of spam and fraud by making it much harder for malicious senders to disguise their identity (more about this on the SPF Website).
An SPF TXT record is used to verify the validity of a mailserver. If a mailserver is on the SPF trust list, mails sent from that server will have a higher (better) rating, and will be less likely to end up in a spam folder.
In order to trust Signicat to send mail on behalf of “testdomain.com”, you can include the following TXT record in the domain’s DNS:
v=spf1 a mx include:_spf.signicat.cloud
Any suffixes or additional SPF mechanics (like -all or ~all) will vary from DNS to DNS — you might have to research this if you don’t already know.