Well, in simple terms, IAS is not really a speed, although we call it one. It is a measure of how much air resistance you are meeting. We calibrate the instruments so that, in a standard atmosphere at sea level, the amount of air resistance is expressed in the same units as the speed you are going at.
As you climb, the air gets thinner. There are less air molecules per cubic metre. To experience the same amount of air resistance (that is, to keep the IAS the same), you have to fly faster through this thinner air. That faster true speed is your TAS.
Normally, you don't have to accelerate to maintain your IAS. But by maintaining your IAS and being in thinner air, you will get a higher TAS.
Hope this helps.
All the best.