Translation:I am eating lunch right now.
I believe 正在 emphasizes "in the process of". For example, if someone calls you during lunch as you're eating, you might tell them 我正在吃牛饭 (I'm in the middle of eating lunch). But if your boss catches you walking away from your desk and asks where you're going, you would instead say 我现在吃牛饭 (I'm having lunch now), but you can't say 正在.
I don't have any concrete grammar rules to call on, but I would say I tend to use 正在 when I'm emphasising the action (kind of like saying "I'm just in the middle of eating lunch"), and 现在 more to emphasise the time (like "what are you doing?" "I'm eating lunch now, and later I'll go back to work"). That said, I would say they're pretty close to interchangeable - can anyone confirm?
Duolingo seems to put too much emphasis on there being a clear distinction between the use of 在 and 正在, whereas the reality is that in many situations they are interchangeable.
If you wanted to stress that you are eating 'right now', which sounds very forceful in English, there are other structures you could use in Chinese.
正在 before the verb and just 在 before the verb are almost exactly the same thing. They both make a continuous verb.
正在 puts a little more emphasis on the idea that the action was in progress at one particular point in time.