To save a post, tap below it. When you save a post, it's visible to you from a private section of your profile.

To save a post to a collection, tap and hold . Then, select a collection you've already created or tap to create a new one.

To see posts you've saved and the collections you've created, go to your profile, tap and then tap Saved. Only you can see the posts you've saved. When you save someone's post, they're not able to tell that you've saved it.