Translation:There are seven chairs.
Its used to count things. Like, for instance, when you're simply talking about number seven it is なな or しち。 But when you're counting an object (like a chair, in our case), it changes into ななつ。And there are also many other different ways of counting things that add other words in front of a number.
In Japanese (and other languages in the area), you cannot say five people, two coffees, three hotdogs.... You always have to say five PIECES OF people, two CUPS OF coffee, there PIECES OF hotdog. These words are called measure words or classifiers. You can find equivalents to them in English and other Indo European languages as well, but on Japanese (and other languages in the area like Chinese, Korean, ...) you almost always have to add that little measure word.
Now this little つ is pretty universal for many things, but in Japanese you have quite many other measure words and have to learn which one to use when.