ADS-B data latency, means the time between aircraft onboard GPS catches its own position & other information and when this information is in final use, like on ATC display. For example, if the latency is 2 seconds, it means the aircraft position information you get is actually 2 seconds before.
Technically speaking, different information has different latency. GPS position and velocity have different latency. Very complicated. So for a simple explanation, we’ll take GPS position and ADS-B ground-to-air surveillance application as an example.
I'll list all the latency below.
1. GPS receiver catches aircraft position, time T0;
2. GPS receiver output position to onboard data bus, time T1;
3. Position data from GPS receiver arrives at ADS-B transmitter via onboard data bus, time T2;
4. ADS-B transmitter transmit the position data, time T3;
5. ADS-B position data arrives at ADS-B ground receiver, time T4;
6. ADS-B receiver output ADS-B position data, time T5;
7. ADS-B position data arrives at ADS-B display system, time T6.
The whole latency is T6 minus T0.
Please keep in mind that this is a very simple explanation of ADS-B latency. The real situation is far more complicated.