Basically, some companies pay a dividend to return excess money to shareholders; the company doesn’t have anything better to invest in like growth, and the shareholders get some money without having to sell shares.
A dividend ETF will pick an index like S&P 500, and buy the companies that pay dividends in that index, and pay the dividends to its shareholders. So, like the index, you get a more balanced dividend better than some and worse than other stocks in the index, but you reduce the risk of one stock cutting its dividend and screwing your income stream.
You also don’t get the same return as the index ETF, since you won’t own the growth companies that usually contribute the most return.