Okay kiddo, imagine there is a giant pool of jellybeans in a jar. Now, if you want to know exactly how many jellybeans are in the jar, you would have to count each one of them, right? That's really hard and time-consuming! But what if we don't need to know the exact number, we just want to know how many jellybeans could be in the jar? We can estimate that by measuring the size of the jar and assuming how many jellybeans can fit per cubic inch or centimeter. That's kind of like how oil companies estimate how much oil is in a potential oil field without having to drill down and count each drop of oil.
When we talk about "oil in place," we mean the total amount of oil that could be in an underground reservoir or oil field. To figure that out, geologists and engineers study the rocks and the way that oil is trapped inside them. They use fancy tools like seismic imaging or rock samples to figure out how much oil can fit in a certain area of rock or underground formation.
But remember, just like with the jar of jellybeans, we can't know the exact amount of oil until we actually start extracting it. So "oil in place" is just an estimate of how much oil could be down there, waiting to be found and brought to the surface.