Corporate welfare is any financial benefit purposely granted by government to a specific business or class of businesses and that is not generally available to all businesses and taxpayers. In other words, government officials use your money to pick which businesses will win and which businesses will lose.
Corporate welfare either lowers the costs of doing business or increases revenue for a business without its customers providing the revenue, relative to other similarly situated businesses, broadly speaking. So, while large corporations receive hundreds of millions of dollars in government handouts, similarly situated mom-and-pop shops are left weakened.
In short, corporate welfare increases the profitability of a select business or select class of businesses through government action at the cost of other taxpayers.