What does the person icon mean in iphotos?

That means that the photo is in your Shared Library (as opposed to your Private Library, which would show an icon with only one head).

Could someone else have access to your machine who might do this?

Phone or Mac?

What is iCloud Shared Photo Library in Photos on Mac? - Apple Support

Use iCloud Shared Photo Library on iPhone - Apple Support

You will only be seeing the Shared badge on some of your photos, if you have enabled the hared iCloud Photos Library in the iCloud Settings for Photos. Are you asking about Photos for Mac or Photos on an iPhone or iPad?

Have you enabled the Shared iCloud Photos Library on your device? When you set up an iCloud Photos Library on one of your devices, you can enable that now photos will beshrew automatically, straight from the camera. Your iCloud Photos library might be set up this way, so you are sharing your new photos automatically, if you cannot remember to share them.

I find it very confusing. Some of my photos in the library has this icon while others do not. I have not shared any of them with anyone, so why do they show “shared”? And why do they all appear in the same library? And why are only some photos (apparently the non-shared) showing up on my ipad?

What happens if I move all marked “shared” i.e in “Shared Library” (whatever that means) to “Private Library”?

Thanks for the answer. These observations are on iphone. I still fail to see the point of a shared library as part of the private library (using some sort of tagging) when we already have the ability to make shared albums, which (to me) seem to have the same functionality - while being safer due to its clarity.