The certificate that you purchase to secure your web site must be digitally signed by another certificate that is already in the trusted store of your user's web browsers. By doing this, the web browser will automatically trust your certificate because it is issued by someone that it already trusts. If it isn't signed by a trusted root certificate, or if links in the certificate chain are missing, then the web browser will give a warning message that the web site may not be trusted.
So browser compatibility means that the certificate you buy is signed by a root certificate that is already trusted by most web browsers that your customers may be using. Unless otherwise noted, the certificates from all major certificate providers listed on our website are compatible with 99% of all browsers. For more details about a specific certificate provider, see here.