SSL certificate uses a public key system, which uses a pair of mutually matching key pairs to encrypt and decrypt data. Each user sets a specific private key known only to himself, and uses it to decrypt and sign; while sets a public key made public by himself and shared by a group of users, for encrypting and verifying signatures.
Since the key is only owned by the owner, it can generate an encrypted file that no one else can generate, thus forming a digital signature.
SSL certificate is a file that is digitally signed by a Certificate Authority (CA) and contains the information about the public key owner and the public key. The simplest certificate contains a public key, name, and a digital signature from a Certificate Authority. Another important feature of digital certificates is that they are only valid for a specific period of time.