At Namecheap, you can buy SSL certificates provided by Comodo (now Sectigo) Certification Authority. This CA is recognized by 99% of web browsers. This guarantees that browsers will not show security errors when connecting to a site secured by Comodo (now Sectigo).
The certificates can be issued only for Fully Qualified Domain Names (FQDN), also known as common names of a certificate.
Example of FQDNs:
For an SSL certificate to function properly, the common name of the certificate must match the URL of the site you are trying to reach. Otherwise, a security alert from the web browser will appear indicating that the name on the certificate does not match the name of the site. If the secure site is reached via https://www.mydomain.com/, but the common name of the SSL certificate is mydomain.com, this will initiate the name mismatch message.
All certificates that Namecheap offers provide up to 256-bit SSL encryption strength that is supported by most of modern browsers (the strength of SSL encryption depends on a browser that the site visitor uses and the web server where the certificate has been installed).
SSL certificates are divided into three groups by the validation type. The difference between these certificates is in the way how Certification Authority validates orders prior to issuance.
Domain Validation Certificates
Organization Validation Certificates
Certificates with Extended Validation
Also, the SSL certificates can be divided into three groups by the number of domains/subdomains secured:
Secure one domain or one subdomain. Whatever the common name is specified in the certificate, the www. version of it is also secured by default. Same for the www.example.tld specified in the common name: the non-www. version of it will also be included to the certificate.
Secure one domain and all its subdomains of the same level. E.g. the Wildcard issued for *.example.tld will secure example.tld and sub1.example.tld, sub2.example.tld, sub3.example.tld etc. Note: The next level subdomains, like www.sub1.example.tld, will not be included to the certificate with common name *.example.tld.
Secure multiple domains with completely different TLDs and different levels subdomains of these/other domains.
E.g. example.tld, domain.com, www.domain.com, admin.yourwebsite.org, www.blog.example.com etc. The Multi-Domain certificate has three seats included for the default price, and up to 97 extra seats can be ordered additionally. Each domain and each subdomain will take one seat in the certificate, so example.tld and www/example.tld will take two seats.