Research allows one to access new technologies. A library is required to do research and gold is often needed as well. The research is divided into ages. Bear in mind that once you go two ages ahead of the possible research option, it will be locked so it is recommended to research as much as you can before advancing too far.
There are many different types of thing with can be researched.
These are:
- Improve the recource gain
- New Buildings or Units
- Improve the speed of trades
- Improve Units
-Lower building time
-The abilty to build on recource tiles
-The Abilty to build an expansion
-....
Also many of the thing you can research are linked, like a tech tree.