Rosin is a type of resin that you put on the bow hair so they grip the strings with friction, creating the desired sound. You can’t play violin without applying rosin to the bow hair.
When it comes to which type and brand you should use, again, it varies. There are so many options out there, so narrow it down by your personal preference. In general, however, you would want darker-colored rosin as they're softer and stickier. Remember to choose wisely since rosin can also affect how much cleaning you have to do. For instance, powdery rosins are easier to clean than stickier ones.