eCommerce platform is a ready to use software enables you to run your online store.
There are plenty of options in the market. Some of them providing online store to you with subscription model like Shopify, Wix, Bigcommerce. On the other hand, some of them provide free software to you like Woocommerce. In this case, to launch your online store you need to own your hosting or server or use subscription based hosting services coming with these software.
You can check out our articles in eCommerce Platforms category for further information