Now using Ubuntu22.04 seems to have added restrictions. There is no special requirement for setting a password when installing the system, but if you want to
modify the user password, there are restrictions, and the requirement is not less than 8 characters. For a linux system that I use myself, it is obviously unbearable to enter 8 passwords every time I enter it. In fact, there is also a way to modify it, that is, to enter the root modification. There is no limit to root password change.
For example, I want to change the password of user hgg:
sudo su
passwd hgg
Then enter the password twice to change it successfully. Although you will be prompted that the password cannot be less than 8 characters, it can be changed.