Will photos on my Macbook be replaced with what's in the cloud if I turn on iCloud photos?

I have photos in my Photos app on my macbook that are not synced to my iCloud account. I also have photos in my iCloud account online that I access through a web browser. I would like to turn on iCloud photos on my macbook, but I'm worried I'll lose the photos stored in the Photos app on my computer. Will the two libraries combine and sync with each other?