Much of what we know about the Earth's interior comes from inferences that we make based on observations on the surface.

Observations are data that we gather with our 5 senses; sight, touch, smell, hearing, and taste.

An inference is a conclusion that we make based on the observations that we made as well as reasoning from our own experience.  It's a lot like a story that we come up with to explain what we notice in the world around us.