Salesforce Identity is basically an IAM (Identity Access Management) service offered by Salesforce for their users. This service from Salesforce offers various features. The users benefit from Salesforce Identity as they can use one username and password to access everything they need.
If you have heard about Salesforce and would want to discover more about it, check out the Salesforce tutorial. Meanwhile, for those, who are seriously planning to break into the Salesforce domain, I would recommend you join a good Salesforce certification course like the one Intellipaat offers online. Before joining any course make sure that the institute or the course is reputed and is being taught by industry experts and includes projects that are industry-grade. Because practical experience is only what counts to help you grow in your career. Intellipaat ticks all these boxes quite well.
If you wish to watch a video to help you get started, I suggest you watch the following video on Salesforce Tutorial Full Course.