An autobiographical number is one in which the Nth digit (reading from the left) tells us the number of times that the digit N appears in the number. So 2,020 is autobiographical because it is made up of two zeros, zero ones, two twos and zero threes.
These numbers are extremely rare – the only ones are 1,210, 2,020, 21,200, 3,211,000, 42,101,000, 521,001,000, and 6,210,001,000.