I have watched Inception a few times but I can't understand why they all are crazy about that letter which was in that locker? Why were they struggling to get that letter?
The 'letters' are different in different parts of the story. However the dream metaphor of the safe and the document it contains act in a similar manner.
Extraction Dream (Targeting Saito)
Cobb explains how extraction works to Saito at the start of the movie.
The purpose of extraction is to obtain a secret from the target. They do that by providing a (dream) environment for the target that contains something like a safe - a box that you would contain a secret. The target of the extraction then (subconsciously) fills the safe with their secret. Therefore all the extractors need to do is to share the dream with the target, and break into the safe to get hold of the secret, which in the case of Saito's dream is in the form of a letter or document. Cobb then memorizes the content of the letter to get the secret out of the dream.
This seems fairly straight forward when talking about extraction. The idea that a dream-safe would subconsciously be a place you would store a secret seems reasonable to me. Out of universe, it turns the idea of stealing a secret from someone's dream into a heist movie.
Inception Dream (Targeting Robert Fisher)
In the rest of the movie they are trying to do the reverse, planting an idea into Robert Fisher's mind as if it were his own. They utilize a similar idea, in that the final dream environment contains a safe. Here they encourage Fisher to join their team and break into (what he is told) is Browning's dream.
In the final layer of the dream, Ariadne designs an environment that is a hospital which Robert fills with a projection of his dying father. Next to his father is a safe that Fisher has been told repeatedly by Browning/Eames contains his father's will. He has also been told that the will contains a suggestion that Robert should make his own way in life and break up his father's business empire.
They build up this idea in Robert's mind as they go through the dream layers. When he open's the safe, not only does it contain the will but it also contains the the photograph and the windmill from his childhood, very positive 'artifacts' which indicate Robert treasuring the positive parts of the relationship with his father. Eames observing this smiles because he thinks this indicates that he will accept the idea that has been planted in his mind / the idea in the will.
So 'the letter' in the safe is different in Saito's and Fisher's dreams in the movie, and serve different purposes. However they share a common idea that they are a representation of an important idea or secret.