What does [1] mean in the output of any command executed on R command line?

Question

I think the question is self-explanatory.

Whenever we execute any command in R, the output is preceded by [1]. What does that [1] represent?

Answer 1

I'm not sure why this question was only implicitly answered in the comments, but the answer would appear to be that the [1] and every similar number at the beginning of a line indicates that the adjacent element is the nth element printed.

It's not clear to me why this is needed for something as simple as:

a <- "alpha"
a

which outputs

[1] "alpha"

Nonetheless, it appears to be a consistent prefix on any line of data presented on the console.