Every post in WordPress can be filed under one or more Tags. This aids in navigation and allows posts and to be grouped with others of similar content.
Usually tags are displayed under each post. When someone viewing your blog clicks on one of these Tag links, a Tag archive page with all the posts belonging to that Tag, will be displayed. In addition, if using Widgets, the Tag Cloud Widget can be added to the sidebar to display all the Tags used in a cloud.
The tags could be assigned while creating/editing posts. Check screen option at the post editing page to enable the tags block.
All tags could be managed through the WordPress admin panel > Posts >Post Tags.