On your settings page, you have an option to fill in the basic meta tags for your site. It's a good idea to at least supply a description tag for search engines to have a look at, and you may supply as many others as you want to take the time to create. These tags you enter here will be put into most pages. (Do note that you can use conditionals here in order to use complex logic to determine which tags to show.)

Your individual categories may have specialized content. Click the 'edit' button by a category to edit it, and scroll down to the 'header insert' area... here you can supply custom meta tags for the particular category. These tags are used in place of the default ones. (If you do not enter anything into the header insert field for the category, then the site default is used.)