The Fibonacci sequence is a set of numbers in which each number is found by adding the two numbers before it. It starts with 0,1 then uses the addition rule. The third number will be the sum of the first two numbers, 1+0=1, giving us a sequence of 0,1,1 now the fourth number can be found by adding the second and third number, 1+1 = 2. so we have 0,1,1,2 you can extend this out forever to get the full Fibonacci sequence.
A video on the topic by numberphile can be found here;
https://www.youtube.com/watch?v=Nu-lW-Ifyec