What do you mean by downloading software patches

Every so often usually on a Tuesday Blizzard shuts down all of the servers and applies software patches. What is actually happening is Blizzard is adding new items and changing things across the whole game. Sometimes they add new areas to explore and bosses to fight. Other times they change a way a certain class works. They are small enough where Blizzard can do it by letting you download over the internet as opposed to releasing a new expansion dics.