Lets assume you buy chips packet (Preferably lays, so that you have lot of air in it :D).
Now, before opening the bag, i press the bag. That is, i apply force from outside to the bad. Now, newton says, there has to an opposite reaction force. That is, the air inside Lays packet should provide a force on the bag.
Now, the amount of force that the air produces per square meter of the bag is called pressure.
Now, the air outside the bag does not exert the force. It is me who is pressing the bag. So, we have difference in air conditions. That is, the air inside is at a higher pressure than the air outside (Air inside exerts more force that Air outside).
As i press the bag more and more, there'l be a limit at which the bag take all the force. At some point,the bag bursts, That is to say, the force exerted by the inside air becomes too much for the bag to take, so it gives up. The moment it gives up, the air inside, gushes outward. The rate at which this air goes out is the velocity