To some people, the Earth is flat. Mainly because we can't see how it curves from our own point of view. Thanks to our recent tecnology, we have been able to see our planet from satellites. The Earth appears as a perfect circle from out of space. But its not a perfect circle. We have mountains, we have rivers, we have holes, etc. All this gives the Earths platform a different face.
One of the things you can do, only to see for yourself that the Earth is in fact not flat, you can climb up a mountain and depending on the altitude of that mountain, you can see a sligt curve in the Earths platform.
This is just a small preview of why we can mistake the Earth for being flat.