As many of the other commenters are saying it is a sequence of numbers where the next number is given by the sum of the previous two numbers starting with 0, 1 (then; 0 + 1 = 1, then 1 + 1 = 2, then 1 + 2 = 3, then 2 + 3 = 5 ...)

You see the number chain occur naturally many places in nature in the development of seeds or leaves in plants, where the number of seeds or leaves in layers occur as fibonacci sequences, e.g. one layer has 3 leaves, the next has 5, next has 8 so on.

On a funny side note, you can also use it to approximate conversion of miles and kilometers, as 2 miles is approximately 3 km, 3 miles is approximately 5 km