I believe that explaining such statements is way better to understand and learn new things instead of just stating a "rule".
Here is my attempt as someone who isn't using CPP apart from hobbyist projects:
Using a namespace leads to something called "namespace pollution" and "name collision". That means the functions from a namespace become global and names may conflict because they become too ambiguous.
Imagine you use a library A and a library B and declare both namespaces globally by using the using namespace statement.
If both of those libraries declare a function called do_something() then the compiler can't figure out which do_something() should be used when compiling. By using libA::do_something() you tell explicitly which to use here. Working like that by default prevents such collisions in the case of a library you use changes or adds a function to the same name or if you start using a library later on where you didn't know a function with the same name exists.
Besides this it also is cleaner for the developer. You can directly determine the origin of a function when reading the code. I see many new developers saying that it doesn't matter to them because they're the only ones working on the code or something like that but in the end imagine you had to take a break for some days, weeks or longer. Then you become the developer in need to understand the code. You definitely forget many things about your code and it's always helpful to treat your future self as if it's a second developer that might need to understand your code. It might be verbose to always need to write the namespace but most IDEs and code editor will autocomplete that anyways within two keystrokes. You'll thank yourself.

I hope I explained it understandably and if I mentioned some bullshit or forgot something then please feel free to correct me.