The Rule of 2, 3 and 4 is a guideline for opening with a weak two bid depending on the vulnerability.
Rule of 2
At unfavorable vulnerability (red on white), the recommendation is to be 2 tricks shy of a 2-level contract. In other words, opener should have 6 tricks to open a weak two bid.
Example:
AKQT54
5
987
654
This is the classic example of a weak two. There are 6 probable spade tricks, and it can therefore be opened 2.
Rule of 3
At equal vulnerability (white on white or red on red), the recommendation is to be 3 tricks shy of a 2-level contract. In other words, opener should have 5 tricks to open a weak two bid.
Example:
65 KQJT87 843 52
There are 5 probable heart tricks, and it can therefore be opened 2.
Rule of 4
At favorable vulnerability (white on red), the recommendation is to be 4 tricks shy of a 2-level contract. In other words, opener only needs 4 tricks to open a weak two bid.
Example:
JT9874 5 AT8 987
3 spade tricks + 1 diamond trick = 4 tricks. At favorable vulnerability, this hand can therefore be opened 2.