Typo squatting is the art of sitting down on a bunch of different names that come from 'typos' of something legitimate , hoping that someone will make the mistake.
In this case, it was in the context of Python Packages. With Python, we use the central repository (pypi) in order to download the packages.
The author made a typo squatting package for the 10,000 most downloaded Python projects by removing underscores (_) and dashes (-) from the names of libraries.
After two years of squatting, 530,950 installs were done for improper packages! That's an astounding amount of downloaded just from a single type of typo.
Why is this a bad deal? This type of attack is known as the supply chain attack. By poisoning the packages that a user downloads, it creates a compromised environment from the beginning.
In terms of an actual attack, persistence on the system (via install scripts) or the stealing of sensitive data, such as SSH keys, can occur.
Overall, this is a really effective long term attack for an attacker. Honestly, I remember seeing this banner come up on multiple occasions.