Ko-Fi is basically an easy way for people to send you money via paypal. You make an account, link it to paypal, and advertise it to let people know where it is. From there, it defaults to a $3 tip, I think you can change it to a different amount, though. The one thing to keep in mind is when people do send you money, you need to go into paypal and tell it that it doesn't need any shipping information, otherwise paypal will yell at you.

It's a nice way to do things, as people don't need to fill out anything, they just click a few buttons and they're done. You don't need to worry about handing out your paypal address and the like. It can be super handy for commissions, as people can just tip x number of tips to pay for it.