If you are exploring public content and come across a post that really jives with you, you can share this post on your own Activity Feed!
To do so, tap the "..." icon directly below the post. A menu, such as the one pictured below, will appear. Select "Share Post."
After tapping "share post," you must choose where you'd like to share the post. If you choose Record, select your preferred privacy and tap "Post." It may take a few moments for the post to appear on your Activity Feed, under "My Posts."