I'm trying to buy I used iPhone but they all say icloud locked what's I cloud locked mean?

I'm trying to buy I used iPhone but they all say icloud locked what's I cloud locked mean?

Don't do it! That is a security feature (which is a great feature). The person that had it originally will need to wipe it before you can ever set it up as your own (which by the way if its stolen that means you can't turn off find my iphone either, and it will notify them of the phones location whenever its connected to Wi-Fi or celltower).