Set permissions to alu1 home directory to passing (but not browsing) for group members and others.
What permissions are passing and not browsing?
I know I have to do something with
chmod but I don't know the exact numbers I should put in.
I believe they mean that you should set "execute but not read" permissions. This lets people traverse ("pass") the directory, but not list ("browse") the files it contains. Although it's an uncommon arrangement I still use those permissions occasionally at work, which is where I recognised the wording from.
In your case, it means that files and directories inside
/home/alu1 can only be reached by other users if they already know the right path, and they can't use
ls to find out any names they don't already know.
For example, if you knew that every user had a standard
/home/<user>/Public/ directory with
777 permissions, then you'd be able to
cd into that and use it as normal, but you wouldn't be able to see what else people kept at the top level in their home directory. (You might be able to have a guess at some things, of course, but that's a different story.)
If the alu1 home directory is
/home/alu1, and assuming that you want to keep full permissions for the owner, then you can set the requested permissions as follows:
chmod 711 /home/alu1
Alternatively, if you prefer to use the symbolic permissions syntax, you should get the same results by using
chmod u=rwx,go=x /home/alu1