A "letra" is a rather curious document that allows you to write cheques - even substantially large ones - without having an account at any bank. It is similar to a "money order", but requires a bit more effort on your part in order to put it into circulation.
The way a "letra" works is like this: first you must find a tobacco shop - or "estanco" as it is known in Spanish. There you purchase a "letra" document before then depositing the amount of money it represents into the bank of your choice. At the time of deposit, the bank's details will be recorded on the "letra". You, in turn, can then use the letra as a cheque.