What is tanning oil/how does is work?

So I am very fair skinned and typically burn easily. I'm a teen btw. Anyways my family's going to Mexico in a couple weeks and I would love to tan a little. I bought dark tanning oil but honestly I don't really understand how it works! There were some tanning oils with like SPF 4 which seemed so low that I don't understand why it's even there! I don't want to burn, so can I put sunscreen under the tanning oil?! Or does that defeat the purpose?