@jaivats Hi there and welcome to the forum.
A no-deposit bonus (either free spins or bonus money) is a casino's way of attracting new customers to sign up at their casino. All a new customer needs to do is to register at that casino and claim that no-deposit bonus - either by entering a special promo code during registration, or by asking for it from their live chat service after registration, or by simply clicking on the promotion banner when it appears after the registration.
Some no-deposit bonuses are real tricky ones, so make sure you read and undertand the Bonus Terms applied to that offer, Some may even require you to make a real money deposit before you can start wagering on the winnings obtained from that bonus.
And of course, every casino has their own wagering requirements for their no-deposit bonuses, from a low 35x the winning amount, to as high as 99x.
Whatever you do, read and understand first the terms associated with each Bonus offer!