How come searching by index is so fast?

let's say a table with two columns has 100 quadrillion records. and i want to find a record that has column #2 equal something.

if column #2 is indexed it returns the result immediately, but if it's not the computer has to read the entire table so it takes a long time.

my question is how come the computer doesn't have to read the entire table when the column is indexed? how does it know what the result is without reading the table?