From material.io Material design is stated as a visual language that synthesizes the classic principles of good design with the innovation of technology and science.
In more simple terms Material design is a guideline to good design that stays consistent with its principles. Material design was created by Google most likely as a way to more become brand dominant but hey they made it a free for all.
The best way to become knowledgable in material design is at material.io They have handy diagrams and images to emphasize the points on topics like typography and iconography.
Material design was shaped around being fully responsive and accessible to people of all needs. Making it great for apps and websites.
Here you can find tools to create themes, color chooser and icons based around material design.
To get further emphasis on material design, you have Bootstrap as a framework the framework is more layout (grids, columns, tables) based whilst material design is style (colors, fonts, icons).
Material design certainly isn’t compulsory nor MUST it be used but when designing websites and web apps you should check out the color tool because it gives good suggestions on what colors match and what don’t in different layouts.