Flutes are one of the simple instruments and certainly the oldest-known musical instrument, dating far back into unrecorded prehistory. Even children quickly discover that blowing across a hollow grass stem (or bottle) can make a hauntingly beautiful tone. In less time than it takes to tell, they learn that different size of grass stems (or bottles) produce different pitches. Smaller volumes produce higher notes, and larger volumes, lower ones. Closed grasses stems (or bottle) are simple Helmholtz resonators that produce resonate sound as the air blown across the top creates pulses of pressure in the chamber.
To understand how it works, think about how to create a single pulse. Some of the air going across the top hits the lip and goes into the chamber, causing an increase in pressure. The pressure inside builds and pushes back till it overcomes the pressure coming in. Because the pushback has inertia, it produces an overcompensation in the pressure, making lower pressure in the chamber. 
Low pressure inside causes air to rush back in, starting another pulse. Like a weight bouncing on a spring, the air pulses go in and out of the chamber. To prevent the pulses from dying out, the airstream blowing across the top keeps adding a little energy. The pressure pulses generates the sound that resonates in the chamber.
The flute is an acoustic cavity resonator. Air in the pipe of the flute is set in harmonic motion by pressure pulses generated by air being blown across the top of the mouthpiece. The flute cavity is an open cavity. Its size (and resonant frequency) is determined by which valve is open. The larger the cavity, the lower the note. Blowing more air across the mouthpiece puts more energy in. making the sound louder.
To generate sound in a flute, you must blow air across the top at first the right angle and velocity to get the pulses started. That takes lots of skill and practice. Ducted or fipple flutes are much easier to play. Blowing air into the duct directs it across the lip at just the right angle to get the pulses started. Police whistles, recorders, ocarinas, and pipe organs are examples of fipple flutes.