https://meta.stackoverflow.com/feeds/tag/[insert tag name here]
Example: If there is a tag called "hotdogs" I can subscribe to that tag feed with the url:
You can filter questions by tag by adding tags to you interesting tags list (that will provide highlighting for questions that come through matching your interesting tags). By clicking on a tag, you will be displayed a list of questions that match the tag you are looking for.
Another method is to use the search box and search for '[tag name]' (include the square brackets) and that will search specifically for that tag. Searching for '[tag name] my query' will search the questions for you query and limit the results only to those falling within [tag name]