Wildcard SSL certificates are those that can be used for a domain and all of its sub domains. A regular SSL certificate can only be using for a single domain or single sub-domain. So if you have several sub-domains like www.example.com, mail.example.com and community.example.com, each one would require a separate SSL certificate. A wildcard SSL certificate can be used with all the sub-domains for a particular domain.
These SSL certificates cost more than SSL Certificates for a single domain/sub-domain. However, if you have multiple sub-domains it can work out to be more cost effective. In addition to being more cost effective, Wildcard SSL Certificates are also easier to manage, as you only need a single certificate which can be mapped to multiple sub-domains. In the other case, you would need a separate certificate for each sub-domain which can difficult to manage.
For example your wildcard certificate for *.mywebsite.com, can be used for:
and any number of websites.