normal vat invoice

It sounds quite dumb but I really haven’t understood what this ”invoice” means? so I did not understand points below:

1. a vat invoice must be issued when a standard rated supply is made a vat registered business
2. no invoice is required if the supply is exempt, zero-rated or to a non-registered customer.

3. A vat invoice should be issued within 30 days of the date that the taxable supply is treated as being made.