short version
Normally you connect to the internet by laptop/phone wifi to your Wifi
Access point/router and it passes it to the modem. It passes it to you
internet provider till it get to where you wanted it to go.
 Imagine a second wifi access point/router with the same id and wifi
password that is controlled by me.
 Your phone/laptop cannot tell the difference which one it is connecting to.
now I just need to copy all your wifi traffic going though my access
point/router
I will know where you go what you did and any passwords you used and ever
redirect you from
where you wanted to go to where I want you to go and trick you into
installing my software on your
computer.
 VPN is a program that encrypts your traffic from A to B so that I cannot
read it and is difficult or almost impossible to decrypt with out the keys.
VPN's need a server and client to encrypt and decrypt on each side.
many home routers have a built in vpn server and client programs are
available for most any OS and most phones. I use a VPN  both on my
computers and phone for most of my work.
I vpn to work (vpn Server) from home using my desktop as a client and when
not at home I use a vpn to home vpn server on my laptop and phone clients.
I find that most home router vpn server are a bit hard to figure out and
may not be current technology so I recommend to new users and students to
use a old pc  64 bit 512meg ram 10gig HDD will be more that enough and two
network cards with IPFIRE or pfsence is a easy to use home server that
includes openvpn servers and a great way to learn basic routing.
we have built quite a few at the install-fest and would be happy to help
anyone interested.
the next one is Saturday at UAT 40th and baseline 10-4pm