"Current dollars" are prices as quoted in dollar units paid at a particular time. Say that in 2000 a gidget cost $2.00, and the same brand and quality gidget costs $3.00 today. Those are current-dollar figures. In constant dollars, a gidget costs 50% more.

But let's say that there has been 50% inflation since then. In order to compare the actual cost of a gidget there has to be adjustment by 50%. Thus, in constant dollars the 2000 price would be $3.00 today. The effect is that gidgets are the same cost today as they were in 2000, when expressed in constant dollars.

The same sort of computation needs to be made in order to compare the size of a national budget at some remote time to the national budget today.

Simple.