The rule of 7 

The rule of 7 says: Add the number of
cards in your hand and dummy in the led suit and subtract this number
from 7. The result tells you how often to hold up. 

I.e. holding Axx opposite xx you calculate: 7(3+2) = 2 ==> Hold up twice.  
The reason is, if you have 5 cards, the opponents have 8. No problem if they are divided 44, but if they are 5/3 you can cut communication by holding up twice.  
KQx is better than Axx. You can decide not to hold up, because you have a second stopper, if LHO gets the lead.  
If you got 2 stopper in a suit, you hold
up once less than with one stopper. 

Links a got from Google typing:  "Rule of 7" Bridge 
Hold up and ducking strategies  Read this to begin with. 
http://homepage.mac.com/bridgeguys/Conventions/Rule7.html  Definition 
http://homepage.mac.com/bridgeguys/Conventions/Rules.html  All the numbers 
http://www.verobridge.com/Numbers.htm  More numbers 
http://lodemarcos.org/luis/LuisNTHoldup&AvoidancePart1.doc  Great BIL lesson 