Version control is the way you keep updates and changes to your code organized. It's important because many, many, many things can go wrong when you're writing code. This is especially true when many, many, many people are all working on the same code and the right hand doesn't know what the left hand is doing.
That's where git comes in. Git is software that helps with version control by managing all of the changes and merging them into the main piece of software, much like all of Voltron's little lion robots coming together to form one big robot. When you're using git, it's like taking a copy of one of the lions, improving it, and then sending it in to replace the original one. With git, the old lion is stored, just in case the new one doesn't work out so well. Maybe it doesn't play well with the other lions or it's just plain broken. When that happens, git can take the old lion and put it back in until the new one is fixed.
In our Voltron example, all of the lions would have to be in proximity of each other to form Voltron and beat up the big bad guy. It would probably have been a lot more efficient if each of the pilots could have just logged in and sent their lion to help form Voltron from wherever they were. (That would have destroyed the plot of many episodes!) That is what GitHub is for. GitHub allows users to work on the same piece of software without stepping on each others' toes, but from anywhere. It's git, but online. It's pretty handy because you can access and make changes to your code from anywhere you can access the internet and have the same version control that you would have if you were sitting next to your coding buddies, or fellow Voltron pilots.
Now, form blazing sword!