This functionality allows mentioning other users within conversations, so that they are notified of comments, recommendations, and feedbacks.
1- Go to the conversation, press the + in the bottom bar and then click on the Notes icon.
2- Write a note, and when mentioning your partner, press the "@".
3- To send the note, press the green checkmark.
Your teammate will see the notification in the upper right corner of the screen (remind him to have browser notifications enabled).