Bootstrap is an open source set of files written in CSS (or Cascading Style Sheets, a programming language used to dictate how a website or web app looks and works) that covers some of the building blocks of most web apps, such as buttons, tables and forms, page templates, app navigation and even stylistic matters such as typography and color gradients.
The simple and small tools released by Twitter, The file size would be around 300-500 KB so it doesn’t matter anyway. Extract the bootstrap in a directory named bootstrap and place it inside the folder of the theme for which you wish to add the bootstrap.
Once downloaded, unzip the compressed folder to see the structure of (the compiled) Bootstrap. You'll see something like this: