First, not every impact is serious. It depends on the energy released as to how much damage is done. Size is a consideration, small objects impact the Earth every day, and in great numbers. Some are so small that they vaporize high in the atmosphere from heat generated by friction. An object the size of a grain of sand might make a streak of light across the sky, but never reach the ground.
Larger objects can and do reach the ground. In the air they are called meteors. On the ground they are called meteorites. Some meteorites are rocky, others have a high metallic content.
Very large objects, a mile or so across, or even more, bring in lots of energy. Remember that they came from way out in space, and have been accelerating not only by the gravity of the Earth, but also by the gravity of the sun and possibly some of the other planets. Think about dropping a ball. As it falls it accelerates, so the farther it falls the faster it is going when it strikes the ground. Having come so far, these objects are travelling at high speeds when they reach the ground. Their energy is calculated as half the product of their mass and the square of their speed. Energy is conserved, so all of that energy goes somewhere. Much of it goes into heat.
If the object is solid rock it may reach the ground intact. If the object has gas, or something that can vaporize into gas, like water, an explosion above the ground can occur. In either case a crater can form.
A massive explosion, whether it is airborne or on the ground, carves out a crater, throwing debris into the air. The energy raises the temperature of the atmosphere, and if the object is large enough this rise in temperature can be worldwide, causing forests to incinerate. Plant and animal life is lost if an impacting object has enough energy, including humans. Then, the debris that is thrown high in the atmosphere screens out the sun for years, dropping the Earth into cold. This impacts the initial survivors. There will be few plants and animals that will survive such an event. The food chain would be interrupted.
A large enough impact can bore down to the mantle, especially if it occurs where the Earth’s crust is thin. This would expose molten magma, and release toxic gases.