If you are not familiar with the binary system, look it up in google, there are tons of tutorials and videos.
Any number is made of bits. For example, number 14 is 00001110 in binary. Setbit and Getbit allow to access individual bits in a number. Basically, this allows to use a single numerical variable as a small array of bits.
For example, you might have 10 treasure chests in your game and you want to keep track of which chest is closed and which one is open. The usual way to do this is with an array. But you can also use a single global variable ChestsStatus to store this information for all 10 chests.
When a chest becomes open, Set ChestsStatus to setbit(ChestsStatus, chestNumber, 1)
This will set the bit for this chest to 1, which will indicate that this chest is open. Then you can save ChestsStatus variable to local storage, and later retrieve the information about open chests using getbit() expression.