This is called do-support. It's required in a lot of places in English, but this isn't one of them. Here, it's optional.
When you use do in this fashion, it emphasizes that something is true. This is most commonly in response to a negative statement or question:
Q: You don't like my dress, do you?
A: No, I do like it! Ultraviolet looks lovely on you.
It can also be used to set up a contrast with another sort of negative:
Q: You don't speak Japanese, then?
A: Well, I do speak Japanese, but not very well.
Q: How do you like the food?
A: Well, I do like the kale, but I can't eat that much of it because I ate two gallons of ice cream for dinner.
Since this sort of do-support is optional, you could remove do from any of these examples and they'd still make sense. They'd just be less emphatic.
Wikipedia also talks about this sort of use and provides further examples.