Talk time is the officially quoted longest time that a single battery charge will last when you are constantly talking on the phone.
The talk time is highly dependent on the cellular network environment such as the distance to the closest GSM cell tower. Moving fast while talking (as in a vehicle) also negatively affects battery life.
Manufacturers measure talk time in controlled conditions and the quoted numbers are rarely reached in real-life scenarios. These numbers are best used as reference when comparing phones from the same manufacturer because battery life measurement methodology will probably vary from manufacturer to manufacturer.