DKIM (DomainKeys Identified Mail) and SPF (Sender Policy Framework) are two different technologies attempting to beat email spam. Different email providers support different combinations of these. Simplero supports both so you can cover all the bases.
They both work by tying themselves to the domain name that you're sending emails from. So if you're sending email from "email@example.com", then the domain is "simplero.com".
They work by looking for some specific DNS (Domain Name System) records on that domain (simplero.com in this example), that verify you are indeed the likely sender of this email, as opposed to someone claiming to be you.
(Interestingly, the email protocol itself lets you use any email address as the sender, so anyone can impersonate anyone else. I'm sure you've experienced this when getting spam or phishing emails that portend to be sent from someone you know.)
You have to create these DNS records yourself since it's your domain, which can be a bit tricky.
DKIM works by signing the email using a private key, whose public key counterpart is stored in your DNS records. We supply you with the public key, and you just need to add it to the domain you want to send email from. That also verifies the email wasn't tampered with in transit.
If you send email using other providers as well, for DKIM there's no overlap. Each key lives on its own subdomain, and you can add as many as you need.
SPF works by specifying which email servers are allowed to send email on your behalf. For this, you need to include the servers that Simplero uses to send email on your behalf.
For SPF, it's a bit more complicated. If you're using other providers alongside Simplero to send using the same domain name, you're going to have to study the specification a bit, and figure out how to combine what Simplero needs and what the other providers need, and make it one combined string. It's not super complicated, but it does require you to read and think a bit.
Or you can get a technical person to help you out with it. It can seem daunting, but really, it's not all that complicated once you give it a little time.