So there are things called elements, all things on periodic table, like hydrogen, oxygen, carbon, chlorine, magnesium, etc. All of this can be further broken down into proton, neutrons and electrons, but that's slightly off topic. So when elements bond together they form compounds or molecules, like water (made up two hydrogens and an oxygen). The bond is a sharing of electrons that keeps these elements together. When you have several different kinds of substances together you get a mixture. so think of salt water. salt (made of sodium and Chlorine elements bonded into crystals that we call salt) dissolves in the water but they don't combine to form a new compound or molecule. Through other processes you could separate the salt and the water. This is reversible physical combination of two molecules that only change the state of salt crystals. A mixture is just a combination of things, like air (made up of different but distinct gasses), or toothpaste, where compounds are physically mixed together but stay fundamentally what they started as. if there is a chemical change, like hydrogen and Chlorine gas reacting to form hydrochloride acid, then it is known as a new compound.
hope that helped.