A letter bank is like a big box of letters that we use to spell words. Imagine you have a big box of letters with every letter of the alphabet inside. You can use these letters to make lots of different words, like "cat," "dog," or "mom."
The letter bank works a lot like a piggy bank. When you want to buy something, you put your money in the piggy bank and save it up. When you want to spell a word, you pick out the letters you need from the letter bank and put them together.
So just like a piggy bank can hold all different types of coins, a letter bank holds all different types of letters. You can use the letters to build all sorts of different words, like building blocks!