GPS receivers use almanac data to predict which satellites are nearby when they're looking for GPS signals. Almanac data includes a set of parameters for each GPS satellite that can be used to calculate its approximate location in orbit.
Using almanac data saves time by letting the receiver skip looking for satellites that are below the horizon.
GPS satellites include almanac data in the signals they transmit to GPS receivers. Although variations in satellite orbits can accumulate with time, almanac data doesn't need to be highly accurate to be useful. Data collected before your receiver was last switched off may remain usable for weeks or months.