Bootstrap is a UI library that provides components you can use and reskin. It does not give you design rules on how to combine the components, ie. it does not say what is "right" or "wrong" in terms of the final design. It just says "use this code and you'll get this widget". For example, it gives you a bunch of buttons - you pick one and use it. There are some basic implied semantics for things like 'danger' buttons, but it doesn't define what is 'dangerous' in your application.
Material Design on the other hand is a design language that defines a set of rules/guidelines on how to produce a design that is coherent for Google products. It is an expression of Google's brand - what makes a Google product look like a Google product? It provides a design philosophy to explain the rules you need to follow. It also provides a UI library that helps you implement the design guidelines. So to go back to the buttons example - for a given action, Material Design tells you what kind of button to use, how it should be placed, how it should move/animate, and so on.