Table and Entity Level Scripts can be thought of as a bit like SQL triggers. They allow a developer to create scripts that are run when certain database interactions take place. They are defined as javascript and stored in meta data. The scripts are then executed when a record is inserted, updated, or deleted on a specified table in CRM.
Below are different articles that have been written over time which cover creating these scripts.
Objects available in Table and Entity level scripts
Designing Code for re-use across Table and Entity Level Scripts
Typical Table Level Tasks
Cascading a change through the system
Dynamically setting and changing data in an insert or update
Using Table Level scripts in the context of Administration
Debugging and troublshooting scripts
Entity Level scripts with Rollback