When connecting from Country A to Country C via Country B (i.e., flight goes from A to B, then another flight from B to C), some countries offer "transit" -- they do not require clearing Country B's customs or security, and simply allow you to step off one plane and onto the other.

The US is not one of those countries. When connecting between two other countries via the US, a passenger must clear US Customs, then US TSA security procedures before being permitted to board the second flight.