I hear this all of the time. Under Florida law, there is actual and constructive possession.
Actual possession is when the illicit drugs are on your person. In other words, the drugs are in your pocket, in your sock, or in your hand. Actual possession is an easier case for the State to prosecute. There may still be issues with a warrant or a stop that can lead to a suppression of evidence.
Constructive possession is where it gets tricky. Constructive possession means a person exercised control over a substance. Mere proximity does not establish intentional control. In order to prove "possession" the state must prove beyond a reasonable doubt that the defendant 1) knew of the existence of the drugs and 2) intentionally exercised control over the drugs.
Let's take the following scenario. You are the passenger in a car with a friend. You are sitting in the front, passenger seat. Unbeknownst to you, there is a make-up bag underneath your seat with .5 grams of marijuana in it. There is a traffic stop and subsequent search. The driver of the vehicle gives permission for a search of the car. You and the driver are arrested for possession of marijuana and paraphernalia.
It is important to note here that Florida allows for joint possession. Two or more persons can possess the same drugs. In other words, two or more people can be arrested, prosecuted, and found guilty for the same 0.5 grams of cannabis (or even something smaller like the same roach).
Why are you being arrested for something that you had no idea was even in the car? "Those are not my drugs," you tell the officers. But they won't listen. You find yourself being handcuffed, booked, and your mugshot being taken. Now you are facing up to a year in county and a driver's license suspension. Charges like this are frequent.
It is imperative that you have a criminal defense attorney beside you to help defend you against these charges. An experienced criminal defense attorney can help bring that reasonable doubt that you knew of the existence of the marijuana in the first place should your case go as far as a trial.