A Geocache (or Cache for short) is a container (mostly consisting of a tupperware box or similar, though the size may very significantly) hidden somewhere in an publicly accessible place. It's GPS coordinates are recorded on internet sites such as geocaching.com or opencaching. Inside the container there has to be at least a log book which is signed by every person (aka Geocacher, Cacher) that finds the cache. The activity of searching for a cache is called Geocaching or Caching.
Caches come in different flavors, the three most common are as follows: There is the "Traditional", where one gets the coordinates directly from the web page. A "Multi Cache" consists of several stages; at each stage the cacher has to collect informations to find the "Final", the actual cache containing the log book. To find some caches, called "Mysteries", one has to first solve a puzzle or answer some questions that lead to the Final.
After finding and signing the log book inside the container one can also post a find notice on the web site to let other cachers know that one has successfully found the cache.
The first geocache was placed in 2001, shortly after the American GPS satellite system was made available for every day use. In recent years a huge and still growing community has formed around the hobby of geocaching.
In the Tomb Raider Games
In Tomb Raider (2013) Lara can find small secrets that look like Bison Tubes, a small container that was originally made to keep small things dry in an outdoor environment, and are called GPS Caches. These strongly resemble geocaches.
In Real Life
There are over one million active caches all over the world. Some of these are Tomb Raider themed. A quick web search reveals about 800 caches containing the words "Tomb Raider" or "Lara Croft".
Many of these caches are Mysteries, for example the one placed by the "Tomb Raider Meeting Team" in Bad Grund, Germany, called A Survivor Is Born - Alle Jahre wieder!. To find this geocache one has to answer a series of questions about the game, like "How old is Lara Croft in the 2013 reboot game?" All questions concerning this cache can be answered trough the use of this wiki or it's parent web site tombraidergirl.com.