If you build a company, when negotiations happen an angel investor or a VC might want to write you a check for a percentage of your company.
Generally an investor will pass you a term sheet, this is something that is non binding but is a layout of how the company will change, what the investor expects and if it is a good term sheet will motivate the startup team to progress the company.
Some more intesting info on term sheets:
Some terms you should know about term sheets:
Types of investments an investor will make: