What is meant by drug tolerance?
It basically means, that the body gets used to certain drugs and that you need to raise the dose to reach an effect. This is for example true for a number of anesthetics or pain medications as morphine.

Permanent application leads to a down-regulation of the drug receptor (for morphin this is the mu-opiod receptor) and the activation of counteracting signaltransduction pathways. To still get an activation you will need a higher drug concentration.
For more details have a look at these articles: 


Opiate tolerance and dependence: receptors, G-proteins, and
antiopiates
Cellular neuroadaptations to chronic opioids: tolerance, withdrawal
and addiction