There are a few different things that can happen when you reduce taxes on large companies. One is that the company may invest more in their business, leading to growth and more jobs.
When you reduce tax on the biggest companies, you give them more money to invest and grow. This can lead to more jobs and higher wages for their many workers, which encourages more economic activity overall.
Another possibility is that the company may use the money saved from taxes to buy back stock, which can help boost their stock price.
Finally, the company may use the money to pay down debt, which can improve their financial stability.
However, it can also lead to increased inequality, as the the company may simply pocket the extra money. It is important to also incentivise growth when reducing taxes on the biggest companies, to make it punitive to pocket it and rewarding to use as a tool for growth.
For the inverse effect, see this FAQ: What happens when you raise taxes on the biggest companies?