Keywords are generally intended to get the attention of people using the search engines for a particular "thing".
So, someone looking for information on dog collars might do a search on "dog collars" which the engines would see as "dog" + "collar" (unless the user literally put "dog collars" inside quote marks).
So, if your blog has to do with dog products and sold dog collars, you would want to have "dog" and "collar" featured in your blog posts, maybe in your template, and as text for your pictures.
Now when someone searches on "dog collar" perhaps your site will come up in his/her search. But it will depend on your site's "ranking" as to where in the search result your site is placed.
You should do a Google search on the topic for more information. The Google site itself has a wealth of information on search key words and placement.