What's the difference between Current Period and Current Period Roaming?

I just got internet on my iPhone today and I want to keep track of how much data I'm using since i only have 1 GB. I've been connected to my Wi-Fi all day today and the current period says 368 KB while the current period roaming says 0 bytes, does the current period include what I've done through Wi-Fi? Does current period roaming show cellular data usage only or?

Also, if my phone is on lock screen I see it change to 4G and back to Wi-Fi when I click my home button, does my phone use cellular data when on lock?

I imagine the current period roaming shows the amount of data you have used while roaming (presumably it's 0 bytes, since you haven't been roaming). Also, the current period for data usage doesn't include Wi-Fi, so if it says 368 KB, that means that you have used 368 KB of data. When your phone is sleeping and not connected to power, it can switch over from Wi-Fi to save power, so that's probably what's happening.