It's your choice.You can add a general site-wide description to the template, in which case it will apply by default to every post - and willl potentially appear in search results for every post. So, if your blog is about cooking, and one of your posts is a recipe for apple strudel, and if a visitor searches for apple strudel recipes, the description will appear as somehthing like "My favourite recipes, handed down to me by grandmother" - or whatever other description you chose. The point is that It won't be related to specifically to apple strudel.
If that's not what you want, you can add a separate description to each page. You do that from the Post Settings panel, to the right of the main edit box for the post.
The above applies to the Description meta tag. You also asked about the Keywords meta tag. You can add that to the template if you wish, but as far as I know there is no way of adding it to individual posts. However, the Keywords tag doesn't serve any useful purpose, so that's not really a problem.