lower taxes mean business gets to keep more of their money, with that money they generally invest in growing the business with new technologies, capital investment, expanding markets etc....all these things need more people to make the business successful.....the more people you have paying the lower taxes the more revenue goes to the government.
A gross simplification of this would be let's say that you have 100 people paying a tax of $100, they pay a total of $10,000. Now if the tax rate were $110 and only 80 people were paying it the total tax collected is only $8800.....now multiply that by 130 million people (give or take a few) and pretty soon you are talking about real money.
That's the theory that has been proven to work under Kennedy and Reagan...tax rates go down, revenue goes up. I know it sounds kooky, but there it is.
edit....oh, and people who get to keep more money invest it in things like savings, durable goods and even entertainment...all of which keep the economic wheels properly greased.