Bootstrap is a responsive framework that is deployable on any website no matter whether it is dynamic or static and is not dependent on the CMS used. You can read about it on the web and discover for yourself it's pros and cons. It's been widely adopted and generally is well thought of; It uses a grid that is achieved using CSS and if that clicks for you it will be easy to understand.
Some people object to frameworks because of "code bloat" and indeed some are critical of things like WordPress for similar reasons. The more it tries to appeal to the masses the more it has to do whether you need it to or not.
You can carry on designing WP sites and never need Bootstrap or any other framework. Each time you move on to another product you will need to get used to slightly different ways of working. For instance, my adopting Reactor was quite a challenge for me because the way it uses wordpress was new to me. But I want to learn that stuff and it felt useful to do so. But it is important to stress that WordPress is the same whatever way you go and all of its capabilities should still be available to you depending on your ability with coding.
I am no expert and others will surely have different views. But I like Bootstrap and I have enjoyed working with Reactor