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