Paid-up capital is the money (‘capital’) collected by a company from issuing shares of their stock. In other words, its money raised from issuing and selling stock.
Paid-in capital is not money borrowed, but rather money invested in the company by shareholders. A company will generally issue shares of stock with a par value and an offer price, and paid-up capital represents the difference between total dollars invested and par value of the shares.
Shares that are bought and sold on the secondary market do not increase paid-up value, as those transactions occur between shareholders and do not flow back to the company in the form of new capital.