The "@" mention is a powerful feature, but use it wisely. Tagging members on every single post, or tagging multiple members at a time is not a good practice. Make sure your post is relevant to the people that you tag, otherwise you may find yourself getting reported to the moderator for inappropriate use of this feature.
If you are commenting on a post or an article and want to address your question to a specific person in the thread, type the "@" symbol followed by the name of the user. Click on the user you wish to tag.
The tagged user will receive a notification that they have been mentioned, and it also lets the community know who you are addressing in the thread.
Most users don't know about this feature, but you can save some time when writing a post that refers to a document by typing the "@" symbol followed by the name of the article. Select the article you want to reference and it will appear as if it has been linked.
Check out the other Compass tutorials to learn more about community features.