Something that's oblate is rounded, but instead of being a perfect sphere, it's flattened slightly on the top and bottom. Although we imagine the Earth to be shaped like a basketball, it's actually slightly oblate.
In the case of our oblate home planet, its poles are a bit flat because of the effects of gravity as it rotates. To get a good mental image of an oblate shape, picture a lentil — while a pea is spherical, a lentil looks like it's been squashed flat. The opposite of oblate is prolate, like an American football. Though it shares a Latin root with the adjective, the noun version of oblate has a completely different meaning, "dedicated religious person."