Bootstrap is an industry leading Front-end development platform. And now in human language: Bootstrap allows us to develop a website or web application design with mobile devices being top of mind. This philosophy is sometimes referred to as 'Mobile first' development. This, by no means, implies that the look and feel on other device types are neglected. It is just better practice to consider the most difficult (smallest) screen sizes first, to ensure that whatever is being designed, it will display properly on phones and tablets.
Bootstrap is used by industry leading platforms. It really is the only way to go if you want to create immersive visual experiences in all sizes. Have a look around at the official Bootstrap website.