Certificates are cryptographically signed by something called a Certificate Authority (CA), and each browser has a list of CAs it implicitly trusts. These CAs are entities that have a set of cryptographic keys that can be used to sign any certificate, often for a fee. Any certificate signed by a CA in the trusted list will give a lock on a browser, because it's proven to be "trusted" and belongs to that domain.
Whenever you use internet for to buy something or just for check mails you have to use your information like name, address, Email, Credit card number or banking details for transaction or for log in/ sign up information. To secure this data SSL certificate is used in simple word SSL certificate take data from user’s browser to server securely by using encryption.