It is about increasing money supply.
The government can do this in two ways: print more money and spend it themselves or take less money in taxes from both corporations and individuals.
Notable are Pres. Kennnedy and Reagan who reduced taxes substantially. In both instances a number of things happened, however, we shall focus on Reagan as this is germaine to your question in current history.
Revenues to the Treasury increased because individuals were more likely to pay their tax liability than to attempt to avoid it.
The cost of money went down because there is more money available for loans and investment and as with any other comodity, the more of something there is, the less value it has.
Alone, this fiscal policy was not enough in 2003. Why? Because it was not coupled with increased private sector spending like what RR did in the '80.
That spending reduced the opportunity cost to business to assume risk and provided for tremendous ecomomic growth throughout the '90s.
The success or failing of the 2003 tax cuts were not solely of the policy itself. In fact, the policy was working until the Federal Reserve systematically and relentlessly raised the cost of money through the raising of interest rates.
In effect, the FR actions shrank money supply and cancelled out the stimulus provided to the economy through the tax cuts.
The current FR actions coupled with tax cuts will fully work through the economy in two years or so, making the next president look like a hero.