Plugins are additions to the CMS that can provide extra functionality.
Some plugins provide critical functionality per the specifications of the site and thus are required. Other plugins save large quantities of time, and/or provided functionality beyond our capabilities.
Some plugins are maintained by the Craft community free of charge. It is up to the original author to decide the cost.
Some plugins have an optional re-occurring (yearly) maintenance cost. If a plugin has a yearly maintenance cost it is to help facilitate future development and plugin updates. Paying for the updates will allow the CMS to grab the latest updates (of the selected plugin) for as long as you pay the upkeep cost. If you wish to stop paying you can at any time, with the only repercussion being you can not update that plugin. If at a later time you do need an update you start the update payment cycle again. Initial purchase of the plugin should come with 1 year of free updates. Not paying the cost will never disable the plugin. These mandates have been established by Craft CMS as part of their TOS to plugin developers to ensure users of the CMS (you) never lose functionality of the (your) site.
Plugins can be installed (and purchased) through the Craft > Plugin Store. Some plugins require implementation from development. Talk to development before adding new plugins.
Plugins installed on the site at launch are considered dependencies for the site, and thus are critical. Removal of a plugin may have negative impacts on the site.
Go to Craft > Settings > Plugins.