For a GPS receiver to work properly, it must know the orbital data of the satellites, so it knows what satellite to expect and where and when

When the GPS leaves the factory, it has a basic orbital data set inside it

When the GPS is put into service, it downloads the almanac from the satellites, and then it has much more accurate knowledge of the satellite orbital data.

So onece the almanac is loaded, the GPS will be able to find and lock on to satellites quicker.

Without an almanac it will still work, but take longer to acquire the satellites.

Jochen