Biochemistry is a mix of two important sciences, biology and chemistry. So, let's look into these two first:
Biology - Biology is the study of life and living organisms, how the live, adapt, evolve, the processes that occur in organisms, etc.
Chemistry - Chemistry is the study of matter and its properties, how matter changes through processes, how matter forms/separates, etc.
Now, if we mix those two definitions into one word, we'll get biochemistry, which can abe defined as:
The study of the chemistry in and of organisms. And that's a basic definition of course, but we can also say that biochemistry is a way of using chemistry to assist in the study of biology.
Hope it helped,
BioTeacher101