how come my photos on my macbook come up with warning signs when i try an open them?

Have your photos been taken with an iPhone? Then you may want to check the Setting for the camera on the iPhone for the image format. If your iPhone should be taking the photos and the videos in the HEIF format, the file format is not supported on a Mac with OS X 10.11. you need at least macOS 10.13 High Sierra to be able to work with HEIF images on a MAc.

• On your iPhone, go to Settings > Camera
• Tap "Formats".

On a Mac with OS X 10.11 this should be set to "Most Compatible". Or upgrade the Mac to High Sierra or newer to be compatible with the HEIF format.

Alternately, you can set the iPhone to transfer the HEIF files converted to a more compatible format. See the details here: Using HEIF or HEVC media on Apple devices - Apple Support

It will depend on the model and age of your Mac. Which model and year is your Mac?

Some older Macs are stuck on OS X 10.11 and cannot be upgraded past this system version. Some Macs could be upgraded to OS X 10.13 High Sierra, and Macs that are only a few years old can be updated to the current system version Catalina.

You may want to start with this page, describing the compatibility with the most recent system version macOS 10.15:

• macOS Catalina is compatible with these computers - Apple Support
• Catalina may not be available or not suitable for you, if you are using older 32-bit Software. Then Mojave might be the better choice, as it still supports 32-bit software: How to upgrade to macOS Mojave - Apple Support

If you cannot get to Catalina or Mojave, try to get to High Sierra at least: How to upgrade to macOS High Sierra - Apple Support

Check Photos' General preference pane and see if you're running a "managed" or "referenced" library?

Which is it?