This is a good question, and the answer is kind of odd. “c”, believe it or not, stands for “combine”, which is what it normally does:
> c(c(1, 2), c(3))
[1] 1 2 3
But it happens that in R, a number is just a vector of length 1:
> 1
[1] 1
So, when you use c() to create a vector, what you are actually doing is combining together a series of 1-length vectors.