What is the meaning of “Failed building wheel for X” in pip install?

Question

This is a truly popular question here at SO, but none of the many answers I have looked at, clearly explain what this error really mean, and why it occurs.

One sour

Answer 1

In my case, update the pip versión after create the venv, this update pip from 9.0.1 to 20.3.1

python3 -m venv env/python
source env/python/bin/activate
pip3 install pip --upgrade

But, the message was...

Using legacy 'setup.py install' for django-avatar, since package 'wheel' is not installed.

Then, I install wheel package after update pip

python3 -m venv env/python
source env/python/bin/activate
pip3 install --upgrade pip
pip3 install wheel

And the message was...

Building wheel for django-avatar (setup.py): started
default:   Building wheel for django-avatar (setup.py): finished with status 'done'