A tag on Steemit is a term assigned to a post). A tag helps describe a post and allows it to be found again by browsing or searching. Contributors can select up to 5 relevant tags for each post. The first post tag determines the main category a post is under and can't be changed after the post is published. Choosing the most relevant tags for a post helps it to be found by users and so potentially contributes to more upvotes.
Tag Spam is using for a post irrelevant tags, especially popular tags.
Spamming in Tags
Spamming in Tags is posting low value content to tags with high frequency.