We call action a unit of financial participation in a company that can be sold to investors. To explain it in a general way, we can say that the total value of a company is divided into units of the same size. Each of these units is an action.
For example, for a company valued at $100 million and issuing 10 million shares, each share will be worth $10.
Once issued, the price of the shares may evolve depending on the value of the company from which they come. Thus, investors who own these shares can sell them and earn a capital gain. Of course, it is also possible that the value of the company will decrease, in which case the price of the stock will go down as well.