The LAW of Total Tricks (LOTT) is a bidding guideline developed by Jean-Rene Vernes. It helps you to decide how high to compete, using this approximation:
With 8 partnership trumps, bid to the 2-level.
With 9 trumps--3 level.
With 10 trumps--4 level.
So, if responder to 1 holds: Q76
, and hears partner open 1, he knows his side has 8 trumps. He won't raise immediately to 2, because that would be a lie. Bids still mean what they always meant and raising to 2 shows 6-10. So, responder has to pass, but he knows that if the opponents now balance (say 1-P-P-2) that he should be willing to compete to 2 since the partnership has 8 trumps. It doesn't mean 2 will make, but even if it is down, that is likely better than defending against 2 (likely making).
Similarly, if responder to 1 holds: Q 8 7 6
K 8 7 6 4
, he knows his side has 9 trumps. He can't raise to 3 since that would be invitational. But, later, if he has to, he should compete to the 3-level (1-P-2-P; P-3-3).
With 5 trumps, responder to 1 will raise to 4 (which is defined as 5 trumps and weakish). Example: KJ876
Over partner's 2-level preempt (6 cards), be willing to compete to the 3-level with 3-card support (9 total trumps) and the 4-level with 4-card support (10 total trumps). After a 3-level preempt by partner, assume a 7-card suit and use the LAW accordingly.
Use LOTT only when you don't have game or slam interest.
Never Lie (1-2 means what it has always meant).
Use the LAW mostly on the 2-3-4 levels (not on the 5-level or higher).
The "LAW" states that on most bridge deals the total number of trumps is approximately equal to the total number of tricks.
There are 4 articles on this website with more details, as well as a brief description of 3 books by Larry Cohen on the LAW:
To Bid or Not to Bid (the classic bestseller on the LAW of Total Tricks)
Following the LAW (sequel to above)
Introduction to the LAW (simplified "lite" version)
Responding to Preempts using the LAW
Responding to 1-Major using the LAW
Balancing using the LAW