What is the meaning of those numbers in the second column after typing "ls -l"?

That's the number of hard links to the file or directory. For files, this will usually be 1, unless you've created additional hard links to it with ln.

For directories, it's 2 + the number of subdirectories. This is because a directory can be referred to either by its name in the parent directory, . in itself, or .. in each subdirectory.

This indicates the number of hard links. This article explain the output of the ls -l command in more detail.