What is supply-side of economics?
The principle supply-side policies that are currently advocated are reductions in tax rates on labor and capital income.  Its claimed that lower tax rates on wages, interest, dividends and corporate income will increase output by increasing the incentives to work, increasing in the supply of labor. 

https://www.philadelphiafed.org/-/media/research-and-data/publications/business-review/1981/br81mjap.pdf?la=en

While supply-side economists expect a little government regulation of the free market, demand-side economists expect a more active government.

Supply-side works by giving incentives to businesses to expand. Deregulation removes restrictions on their growth.

A corporate tax cut gives businesses more money to hire workers, invest in capital equipment, and produce more goods and services.

An income tax cut increases the dollars per hour worked. It boosts workers' incentive to remain employed and creates more labor. 

the supply-side premise is that an increase in supply (i.e. production of goods and services) will increase output and lower prices.