Very annoying thing about my new Mac

It's not a Mac feature, it's a browser feature, and all browsers tend to do this. Like the message says, you got to the page you're on via some kind of form submission (virtually all sites these days are database generated) and so if you want to go back, it may involve having to resubmit that form. It's giving you a chance to think twice about this action, because maybe you just bought something on Amazon, and don't really need to make a second purchase of the same order.

How you managed to never run into this before is a bit of mystery unless this is your first computer or you previously were using a truly antiquated browser like IE6. So, the moral of the story is that however annoying this message might be to see, it'd be far more annoying to deal with some of the potential consequences. However, if you didn't just purchase something online or maybe update profile info on some site, then it's probably pretty safe to hit submit. Just make sure you're a little mindful of where you are from now on.

This is not an OS feature but a browser action and it occurs whatever operating system, and whatever genuine and up to date browser that s being used.

I've just done some online shopping, (a regular grocery shop), and I had to use a password on a debit card check page so the bank could validate my card. When I got to the next page, if I had attempted to return to the previous page, this browser warning would appear. The danger, in my example, is that I would have been debited twice for the same shopping. Or, if I had gone through the procedure again, the bank might consider some scam and refuse to validate the card again. In that case my shopping would have been cancelled.

So, it's an important security warning.

If you are not online shopping, or not entering/updating personal details on a secure web page, when you see this warning, then something may well be wrong. But without details we can't say.

Mark

With time you can learn to recognise pages which are compiled from 'forms', if you want to keep a record of that page either print to pdf or take a screen shot with Cmd-Shft-3 or -4 or the many variations on those short-cuts. (see Keyboard in System Preferences for the basics, search Apple's help articles for more elaborate short-cuts)

It wasn't clear from the post whether you perhaps wanted to go further back than the immediately previous page, in which case instead of simply clicking once on the 'back' arrow, click and hold to reveal a drop-down menu of all the previous pages viewed, with the immediately previous at the top. Simply drag the mouse pointer down the list to an earlier page and you will skip the page which requires a form to be re-sent.