DSC or Digital Signature Certificate is an digital generated encrypted file to represent id of holder.
For example, if you have a PAN and generate an encrypted file according to certifying authority then that will be your digital id or digital signature.
Digital Signature or DSC are unique for each id.
For example, no one can copy your digital id unless id is generated using forged papers.
For GST or income tax we can sign our documents using this encrypted file which equivalents to signing documents with your own hand signature.
In fact in GST you can sign invoices or any document using DSC and it will be valid.
Certifying Authority is appointed by government to issue DSC.
Certifying authorities can further appoint RA to issue DSC on their behalf.
You can generate DSC using any id, but PAN is mandatory in India. You can generate a DSC using your Aadhar number but that will not work with Income tax.
To conclude DSC is an encrypted form your physical id assigned to your and it has unique value representing your signature in digital world.