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HTTP Post method is a way for your to send data or your submission on a target endpoint where it can be received.
To learn more about this, you can check this guide: https://www.jotform.com/help/51-How-to-Post-Submission-Data-to-Thank-You-Page.
Other helpful guides related to this is about PHP Forms: https://www.jotform.com/help/237-PHP-Forms-Explained
One example scenario is that you want to send the submission data directly to your server's database, instead of passing the data via the URL, you can use POST method. However, you should be familiar with programming and PHP to execute this properly as you will be creating your own PHP endpoint that will receive the POST data.
We also have WebHooks integration that will allow you to send POST data without redirecting the "Thank You" page, here's a guide about it: https://www.jotform.com/help/245-How-to-Setup-a-Webhook-with-JotForm
I hope that helps.