This article explains how to use this bookmarking feature in the Instagram iOS and Android apps.
The post immediately saves to your defaultAll postsInstagram collection.
To save it to a specific collection, tap the+ (Plus sign)beside it.

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If you’ve never created a collection before, tapNew collection> name it >Save.
No one can see your saved Instagram posts and your collections are only viewable by you.
Here’s how to find them:
kick off the Instagram app on your iOS or Android gear.

Tap theprofileicon in the lower-right corner of the screen.
TapMenu(three horizontal lines) in the upper-right corner.
SelectSavedto view all your Instagram collections.

All your bookmarked posts live within theAll postscollection in addition to others you’ve assigned them to.