Q:

Number in results from R

In R, when I press return for a line of code (for example, a histogram,) what does the [1] that comes up in the results mean?
If there's another line, it comes up as [18], then [35].

A:

The numbers that you see in the console in the situation that you are describing are the indices of the first elements of the line.
1:20
#  [1]  1  2  3  4  5  6  7  8  9 10 11 12
# [13] 13 14 15 16 17 18 19 20

How many values are displayed by line depends by default on the width of the console (at least in Rstudio).
The value I printed is a numeric vector of length 20, a single number is technically also a numeric vector, but of length 1, in R there is no different concept for both, thus when you print only one value the [1] still shows.
42
#  [1]  42

It's not obvious, for example there is no function of length 2, c(mean, median) is a list (containing functions), but it works like this for said atomic modes (see ?atomic) and usually the classes that are built on them.
You might not always see these numbers on all objects because they depend on what print methods are called, which itself depends on the class. 
library(glue)
glue("a")
# a # <- we don't see [1]!
mode(glue("a"))
# character
class(glue("a"))
# [1] "glue"      "character" 

The print method that is called when typing print(1:20) is print.default, it can be overriden to avoid displaying the [numbers] :
print.default <- function(x) cat(x,"\n")
print(1:20)
# 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
rm(print.default) # necessary cleanup! 

The autoprint (what you get when not calling print explicitly) won't change however, as auto-printing can only involve method dispatch for explicit classes (with a class attribute, a.k.a. objects)
Type methods(print) to see all the available methods. 

