WordPress.com is a propriety blog hosting service provider. WordPress.com is not directly related to WordPress.org which is home of the most popular open source CMS software, WordPress. Even though WordPress.com blog hosting service uses WordPress at its core, there are several limitations to WordPress.com.
You cannot install plugins on your WordPress.com site which is one of the best features of using WordPress. We have heard from many users who switched from WordPress.com to a self-hosted WordPress.org site because they did not want to be limited in terms of what they could do with their site.
You also cannot use custom themes on WordPress.com. On top of all that, you are limited in terms of monetization. You cannot use ad platforms like Google Adsense on your WordPress.com site to make money from it.