In HTTPS, the connection between a browser and a server is encrypted.
If not, anybody on the network could intercept, read and modify the streams (like password, bank account information, and any other data transmitted).
The encryption is made by algorithms - now TLS 1.3 (SSL is actually strongly deprecated) - and certificates are signed by a valid Certificate Authority (CA).
In addition, your company can also emit individual certificates to increase the security of the authentication/encryption.
This kind of certificate must be manually imported and installed in your Internet browser.