I am a newbie, just want an overview of what altcoin is and how can we earn thorugh it?
you can use google to find better answers and faster answers to these general questions:
https://www.google.com/search?q=what+is+altcoinbut in short, altcoins are alternative+coins also known as alternative cryptocurrency which are altenate to bitcoin. you can mine and also trade them on different exchanges just like bitcoin (mining has many differences though).
generally since it is an open source community anybody can copy bitcoin or other altcoins code and make changes to them, or even start their own new project based on others and create a new coin.