Easy way to say it is that earth is a massive object and that smaller object are attracted to larger ones. Perfect example is, if the moon 5 times bigger than the earth, the earth would orbit the moon because the moon would have a higher gravitational pull than earth. This is way the moon goes around the earth because of the earth's higher gravitational pull ratio. Now mess with you just a bit mass (size wise) is only one part to it the other is density. The more density an object has the greater it's gravitational pull is. One of the most dense objects in all the universe is called a white dwarf. It is so small and dense that if it were to magically be placed on the highest point on earth, it would instantly punch right through to the core of the earth destroying the planet. Because of its density it has a strong gravitational pull to it can has the ability to have planet orbit it. That said today no one has found a planet orbiting a white dwarf because to make one destroys the planets in its solar system but the cores of those former planets have been found orbiting the white dwarfs. Now to bring it back to earth, the atmosphere has weight as well and is pushing down on everything. The closer to the core of the planet you are the greater the gravity. Also know that the easiest place to launch something off the planet into space is at the equator due to centrifugal force.
But to answer your question of why we humans don't go flying off the face of spinning planet is, each persons mass is being pulled the earth because of its gravitational pull, the weight of the atmosphere pushin down on us, and the fact that each person lacks the energy/power to push off/escape the combined gravitational pull and atmospheric weight to jump into space. Which is completely possible to do on the moon and is a real concern which is why when you see the astronauts moving in the surface of the moon they make small skip. To big of a jump and off they go into deep space.
There is more to it but this is the basics hope it helps.
