What are vendor prefixes

With CSS3 we got plenty of new cool properties and features like animations and transitions, but not all the browsers support them. They need an extra hint to understand what some of those properties mean. That is why we need vendor prefixes.

-webkit-transition: color .2s ease-in-out;
-o-transition: color .2s ease-in-out;
transition: color .2s ease-in-out;

I suggest you to you use some online tool like autoprefixer that will add those vendor prefixes for you: https://autoprefixer.github.io/

Read this insightful article on how to deal with vendors