A share is an investment in a company. It is a share in the fortunes (good or bad). Sometimes called an equity, it is literally a share in the business.
As a shareholder, you will get a share in the profits of the company (sometimes called a dividend). If the business is doing well, the share price may rise, in which case the value of your investment will rise. On the other hand, if the business is not doing so well, the share price will like fall, meaning a fall in the value of your investment.
Other factors that impact on the share price (but not necessarily the dividend) are: