FediTips , 
You can pin up to five posts to your profile page. To pin a post:
- Log in through your server’s website or web app or a third party app
- Go to post you want to pin, click ⋯ icon, select "Pin to profile"
To unpin click ⋯ again and select "Unpin".
Pinned posts let you expand your profile. Many use them for introduction posts, artists use them for portfolios etc.
Pinned posts federate much more widely and prevent your profile looking blank. More info: https://fedi.tips/how-do-i-pin-my-posts-on-my-mastodon-profile