Eval method in ruby executes string/expression passed as parameter.
Example:
ree-1.8.7-2010.01 > eval(“5+3”)
=> 8
ree-1.8.7-2010.01 > eval(“a=5”)
=> 5
ree-1.8.7-2010.01 > eval(“b||=a”)
=> 5
Its part of ruby meta-programming and not recommended approach unless there is no any alternative to do.
Josh Software 