An Instagram QR code is just the profile URL encoded as a QR symbol. Put instagram.com/yourhandle (or a link to a specific post or Reel) into the generator and a phone camera will open it in the app or the mobile browser. It is one of the cheapest ways to turn physical touchpoints - product packaging, a shop window, a business card, an event banner, a receipt - into followers.
Use the shortest stable URL you can. A handle link is better than a share link full of tracking parameters: it produces a less dense code that scans faster, and it will not break if a campaign parameter is retired. If you are promoting a specific campaign, consider a short path on your own domain that redirects to the campaign, so you can change the destination without reprinting.
Keep the code dark on a light background with a clear empty margin. Instagram's own branding is colourful, but a low-contrast pastel code on a busy printed photo is the most common reason these fail in the real world. Add a one-line caption so people know where the scan will take them.