An SSL certificate is the most common protection against malicious attacks in which other people or machines could pretend to be you. You could call it an Identification Document, but for a domain instead of a person. It lets other internet users validate that all internet traffic coming from and to your domain is actually coming from you and not a hacker.
In the context of using Growbots, your experience will not be affected by not having a valid certificate, but in a very rare occasion of a so-called “Man In the Middle Attack”, a 3rd party might be able to read all the email data that you would be sending via Growbots. Having a valid SSL certificate negates that risk.
More often than not, your domain provider, like GoDaddy or Google, offer such certificates. Growbots can easily verify that and let you know if you don't have it. If your email address in Growbots shows Active as its status, then you have nothing to worry about:
However, if your email account status communicates, that you have invalid SSL certificate, you should contact your email provider and find out why is the certificate invalid. If your email provider doesn't offer such a certificate, you can simply purchase it online via any other provider of such certificates.