What is GIS?
GIS stands for “Graphic Information System” which essentially uses a computer-based map to identify where features were found, what they are, etc. – i.e. your historic environment record .
This is called “modelling” as you are drawing a model of how things may have looked and been laid out during the period being investigated.
From a single find (perhaps parts of a beaker near the remains of a wall in a field that has existed a long time) it is possible to begin to envisage more of the site during a particular period according to the date of the finds. For example whether it was a house, a farm, the edge of a town, how many people may have lived or worked there and the extent of the site (through tithe maps and censuses).
GIS is changing the face and direction of Archaeology .
GIS uses “layers”. Just as you might trace over the outline of a country or town using tracing paper over a map, the same is the done with GIS.
You may want one layer of tracing paper for the country’s outline…
Another for the contours of a hill or surrounding area… 
Another for the transport systems (road, rail, bus-routes, etc.), another for the building outlines, another for where the mains power, water and gas come in…each of these pieces of tracing paper is a “layer” in GIS.
In the last case, the “REAL WORLD” might be an OS (Ordnance Survey) map or even Google map.
Note:
CIfA (the Chartered Institute for Field Archaeology) have policies and procedures surrounding any ‘prospecting’ which include GIS, Remote Sensing, Drone use, etc. . These policies and procedures are applicable in the wider world of landscape investigation .
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