We have all read in school that matter is everything around us. It is anything made of atoms and molecules and matter is anything that has mass and takes up space. If you don't understand mass, think it as the amount of stuff in an object.Matter can exist in 4 different states e.g. solid,liquid,gas and plasma.
All the states depend upon the distance among molecules/atoms present in the material. Atoms are closest when matter is in solid state whereas in state of gas ,atoms are further away from each other as compared to liquids and solids.Changing temperature like heating can change the state of the matter.
Objects keep changing their states due to heat and pressure e.g.water can move in different phases like it changes to ice on freezing & ice can change back to water through heating due to evaporation.