If to tell in Simple words, If you have a business, Acquiring customers using internet for that business, is online marketing of that business.
If you have no business, You don't have any job. But still you want to earn money?
You can earn money by marketing of other people's product or business by using internet, by using social media like whatsapp, facebook, etc. or through your website to get them customers is your online marketing of their product or business.
And because you gave them a customer, They pay you something. This is called Earn money by marketing online.
To give an example
Example one.
You go to a store.
If you buy something in that shop or don't buy anything, but the price of the goods/items kept in that shop is reasonable or cheaper than other shops, if you think it is good, then you send/ask your friends, relatives to go to that shop.
Your friends, relatives go to that shop, if they also think about the goods/items kept in that shop and its price is right or cheaper than other shops, then they buy from there. The shopkeeper earns. But what does the shopkeeper give you? nothing
Example two.
You go to a service provider such as a tailor, a plumber, a mechanic, a doctor,
or an online service provider.
Get a service from the service provider, If you find the services of that service provider and the charges he charges for it to be fair or cheaper than other service providers, you refer or ask your friends, relatives to go.
Your friends, relatives go to that service provider, if they also find the services of that service provider, and the fee asked for providing the service is fair or cheaper than other service providers, then they take the service from there. The service provider earns. But what does The service provider give you? nothing
But if you do the same thing as an affiliate for an internet marketing shop or an e-commerce website or any other website, they will pay you something for giving them customers. This is called making money through online marketing.