Substance is essentially that which is itself such as a tree, a man, a chair. These things are all substances. A human in order to be a human has two elements to it, firstly the conception of what a human is(humanness) and the matter is the stuff by which we are made of(the stuff human bodies are made of). Humans can be different shapes and sizes but we all have something the makes us human, if I did not have arms & legs only gills fins and scales I would not fit the requirement of being human. So we must have a the form of humanness, but also if we did not have matter to be comprised that makes humans real we would also not exist.
Thus it is only by the combination of these two things that allow us to exist, both the form and matter must be present in order to have substance. And according to Aristotle substance is the only thing that exists, that you cannot separate form or matter as being able to exist without substance(so forms and matter do not exist on their own), unlike Plato’s views that forms do exist by themselves and are the most real thing that exists.
Without form matter has no bounds, meaning existence without shape or limits which is simply not possible because that would everything exists at the same time and it is logically not possible. Form without matter could only be one thing, and that would have to be a god, but for Aristotle he disagrees with Plato about the forms. Aristotle argues that without flesh, bone, or material to exist how can a man exist without these necessary conditions? That it would not be a man. There would not be man if they did not have blood, this belongs to what the notion of man is. This is what Aristotle means when he says without Substance(which is the form plus matter) then nothing would not exist since form and matter cannot exist independently of each other.
Credit to The Atlantic for the picture