Rust by default will not allow you to make certain kinds of errora, which is great. But if you are doing something advanced, down at the hardware level [see below], you might need to disable those defaults in order to write the code you need. This is what people mean by "unsafe" -- lacking the normal memory safeguards.
With careful coding, "unsafe rust" or normal C, for that matter, can be free of bugs and safe. But if programmers make a mistake, vulnerabilities can creep in more easily in the unsafe sections.
Is that basically it?