Some textbook authors describe chemistry as the science of matter and change.
Many students find the changes more interesting than the matter but in order to understand the changes we need to know something about the substances as they are at the "beginning" and as they are at the "end".
Matter is something that occupies space and has mass. That's a basic definition from some elementary science course in your past. Chemists go a little further and subdivide matter into two categories: pure substances and mixtures.
Pure substances may be either elements (all atoms alike) or compounds (different atoms combined in molecules of definite proportions and inseparable by physical means).