Depends on what you expect this plugin to do.
This plugin simply redirects your pages from the 'http' protocol to an 'https' protocol for mitigation purposes.
It does not grant you a certificate.
So if you're expecting this to do everything for you, it isn't
If you already know this, and you have a certificate, then this plug in simply forwards users from your http port 80 version to your https version.
This can also be achieved by editing your .htaccess file on your host. Which I am going to assume is all it actually and really does for you.
This plugin will simply insert a rule to any inbound connections and rewrite a rule that converts any incoming traffic from 'http' to 'https' but it does not issue any certificate.
This plugin is used because google looks at your http version and your https version as two complete different sites. Just like they look at your www. and non-www version of your sites. Which is why it's important to define one. This plugin will insert a rule that automatically forwards your http version to your https version providing you have a certificate.
SSL certificates are typically bought, and they have to be maintained and updated. There are several plugins your host can install ON THE SERVER CPANEL (not wordpress), such as Lets Encrypt.. Lets Encrypt is a company that issues free SSL certificates up to a certain extent. There are also several free hosts that do this for you. Ask me if you want a good free host with free SSL.
Most hosts have a section in the cpanel that allows you to: insert the key information of the certificate if you want to maintain and update the certificate on your own. If you have access to the cpanel, and your not sure where to find this section of the cpanel to modify or insert SSL information, you may want to ask your host where it is and maybe they can help you. If you choose to do this and do everything manually, it can get a little complicated depending on how familiar you are with hosting, cpanel and SSL. Lets Encrypt will grant you a free certificate, but it has to be updated and maintained, if you fail to do this, your site will not work.
It's best to use a host that has a script installed on the server that automatically updates all the hosts clients and their certificates and checks for any issues daily. The script automatically keeps up with the expiration dates of all the hosted websites on the server, and when they are close to the expiration date, it files for the new certificate, and also updates all the files and the keys automatically on the hosted space for any of the host's clients, as well as issues for any new clients they may have gained that day.. Which is the best way to go since you don't have to do anything ever.. You can however do all this manually like I said, but it depends on how dirty you want to get your hands.