Following a tag allows you to get alerts on the tag related to a particular service. To follow a tag, select Tags on the header, and hover the tag you want to follow. Select the Follow button. You can also follow tags from other pages by hovering on the tag and selecting on the Follow button.
If you want to receive notifications when a question is posted on the tags you follow, select your Avatar, and then Settings. Make sure you have an email for notifications. Then, on Q&A preferences section, select Receive email when a question is posted on a tag you follow and Follow questions with tags you're following and select Save. You can find more details in the Q&A preferences article.