The dish provides a directional antenna for your wifi signal. It can work extremely well but like most other technical DIY it requires significant knowledge and then good fabrication, wiring and soldering skills. You must find your own "free" wifi signal. The dish will collect the signal better than other antenna types but the real job is directing your own signal back to the "access point" w/enough punch to allow you to be connected. The dish usage I've seen most often is for long range, not necessarily to get internet "free". Good connections can be established over several miles with the right antennas.
But it's not a "no-brainer". "Tricky" would be an understatement. Note the FCC has opinions about what you broadcast, especially how much power. It is dirt simple to exceed their limits and it's hard to get caught. But you can easily wipe out connectability for those in the path of your beam and if you're close
enough, you might prevent others from connecting to the access point. So be considerate and don't SHOUT when a whisper will get the job done.
Rufus