So since the planets all came from our flattened protoplanetary disc, they all ended up in the same plane, meaning the solar system is flat: it looks like a frisbee, instead of like a ball. Except they’re not exactly in the same plane, but they’re close. The drawing above shows the solar system as seen edge-on.
The plane of the Earth’s orbit is called the plane of the ecliptic (because eclipses happen in this plane). The plane of the ecliptic is 1.6 degrees off of the invariable plane, which is a sort of average orbital plane of the entire solar system. If they were exactly the same, it would be 0 degrees off.
The other planets are also fairly close to the invariable plane, with Mercury being the furthest off by far, at 6.3 degrees to the invariable plane. The drawing above is not to scale of course, but you can see how close to flat variations in degrees this small are. Asteroids, comets, and other objects do not adhere to the invariable plane like the planets do, however.