AntcuFaalb said:

I always thought it was about Luke discovering that Vader is his father.

I like BmB's interpretation.

I guess you can read into it that it's also a premonition of that as well but ultimately it's a test in Yodas training which Luke fails. The scene is simple but yet brilliantly done in showing us what dangers lies ahead if Luke isn't careful.

Luke: "What's in there?"

Yoda: "Only what you take with you."

He fails the test because he brings his own fear with him, and Vader symbolizes his fears. It shows us Luke isn't ready to face Vader at this stage, when he sees himself in Vader's mask it tells us what he might become=turn to the dark side. He is also the one who first drew his sword, he takes his weapons with him. It's foreshadowing, he is basically his own enemy. Fear but I guess anger, revenge and impatience can also be atributed to his shortcomings.

"Yes, to Obi-Wan you listen, remember your failure at the cave." Yoda tells Luke when he decides to rush off to save his friends.

The scene is kinda pointless to describe, when it's a nightmarish metaphorical sequence we all understand but it sounds ridiculous when you try to put it into words, especially with my broken english.