A TV works by displaying a series of still pictures, fast enough that you can’t discern them - your brain fuses them together for an illusion of motion.
Each picture is made up of a set of horizontal lines.
720 means there are seven hundred twenty lines making up the picture.
1080 means there are one thousand eighty lines making up the picture.
The “p” means the screen will display all 1080 lines at once.
The “i” means every other line will be displayed at once, then the missing lines will be filled in next: 540 lines shown, then the other 540, letting your brain fuse them into one picture.
1080i is a cheap shortcut.
60Hz means the screen is redrawn sixty times per second.
120Hz means the screen is redrawn one hundred twenty times per second.
The faster the screen is redrawn, the smoother the apparent motion looks. Some people can discern the flicker between redraws at 60Hz; nobody can discern the redraws at 120Hz.
720p @ 60Hz will do, and you will probably not notice what’s missing. Some of us can tell the difference, and it bothers us; most people have no idea*.
1080p @ 120Hz will ensure there is nothing to miss.
(* - videophiles: remember that most viewers can’t even tell that a 640x480 image stretched to 1080p is “wrong”; they won’t recognize any benefit from a proper setup.)
Thank you