As Yannis said, comments already exists, but they can be used everywhere from who has a reputation of at least 50. Who don't have such reputation can just comment on their own answers (answering to other comments), or the questions they ask (which include any answer posted for their questions).
Keep in mind that comments should be used for specific purposes:
- Request clarification from the author
- Leave constructive criticism to help the author in improving the post
- Add relevant but minor or transient information: a link to a related question, or an alert to the author that the question has been updated
If you want to ask the answer a question, it is probable you should be asking another question, except in the case you want to ask the answerer a clarification about something s/he wrote.