Whenever you or your visitors access a website through a secure, encrypted connection, that connection is typically made using something known as SSL. If you make a secure connection in your web browser, you will probably see the address bar light up or some sort of padlock appear. You should also see that the URL starts with https://. The “s” is what tells you the connection is secure.
Both SSL and its successor, TLS function using certificates. Any business site that processes transactions will definitely need secure connections to work. If this is the case for you, then you need an SSL/TLS certificate. Business will often purchase an SSL certificate from a reliable source, such as Verisign or Trustwave, so that customers are assured that their transactions are as secure as possible. Your web host may be able to sell you an SSL certificate from a trusted provider such as one of the companies I just mentioned.
You may also be able to generate a free one from within your control panel, depending on which one your web host uses. If a website is moved to a different domain name, you will need to update its SSL certificate(s). This will prevent your visitors from being scared away by warning messages.
SSL certificates always expire after a fixed amount of time. Your visitors will also receive a warning when his happens, so you should try to remember to renew your certificates before they expire.