We had photos on our computers, on our cameras, and stuck in our phones.
Gathering them all together in one place was a strictly manual affair.
Photo Stream was a one-way trip.

Apple’s Photo Stream shown on a vintage Apple TV.Apple
If you wanted to keep those photos on your Mac, you would have to save them.
It was a precursor to modern cloud sync, where everything we have is everywhere, all at once.
Backing up iCloud
Apple isshutting down Photo Stream on July 26, 2023.

Apple Photo Stream shown on an iMac and an iPad.Apple
Apple’s support article tells you how.
And you should take care of a few other bits of housekeeping while you’re there.
Cloud storage is so ubiquitous and reliable these days that we no longer think about it.

A screenshot of iCloud Photos General Settings page.Lifewire / Charlie Sorrel
But iCloud can stop working at any time.
In fact, Apple has beenhaving lots of cloud-related outages recently.
In general, you don’t need to worry, but you should definitely take precautions.
Then, make regular backups of that computer.
Then, enjoy the benefits of convenient cloud storage, and forget about Photo Stream.