What does ls -alh mean?

ls -alh is the same as ls -a -l -h.

Multiple short options can be combined like this.

Here are the meanings of those options from man ls:

-a, --all
    do not ignore entries starting with . 

-l
    use a long listing format 

-h, --human-readable
    with -l, print sizes in human readable format (e.g., 1K 234M 2G) 

-h stands for human readable.

As mentioned in the comment, you can combine arguments simply like: -alh. The order is irrelevant.

From man ls:

   -h, --human-readable
          with -l and/or -s, print human readable sizes (e.g., 1K 234M 2G)