Question: I want to know what exactly is `lambda` in python? and where and why it is used.
thanks

Answer: 
Lambdas are not anonymous functions. Lambdas are anonymous *expressions*.

They're accessed like functions, but they're not the same thing. Functions allow complex tasks: flow control, variable declarations and lists of statements containing expressions. Expressions are merely one part of a function, and that's what lambdas give you. They're severely limited compared to functions.

Python does not support anonymous functions. For examples of languages that do, see Javascript and Lua.

(Note: It's correct to call lambdas anonymous functions in functional languages, where the mathematical definition of "function" is used, but in procedural languages the word has a very different meaning than in mathematics.)
I would like to explain this with a little layman approach in terms of the usage perspective, since all the technical points are already covered.

A lambda function is just like any other function, only thing that makes it different (other than the syntax and the fact that it can't be reused), is that it is used when we want to quickly write a function which takes an argument and just returns a value using only **one expression**.

**For instance:**
If all that a function does is take a argument and add 1 to it then using a lambda function is a better approach than a normal function.
But if your function requires more than one line of processing before returning the value, then we need to use a normal function definition and then call it.