"Approximately" means "about". Use it when you don't know or don't want to tell someone an exact number. For example, you may hear this on an airplane:

We'll be arriving in San Francisco in approximately two hours and forty minutes.

You can use "approximately" to talk about numbers as well as time:

We have approximately one thousand two hundred students.

"Approximately" is more formal than "about". Use it for written documents and essays, when speaking with customers you don't know well, or in formal speeches.