Gnosticism is way too complex to answer your question adequately on a simple forum like this. However, Jesus himself actually tries to explain in Matthew 13.10-17:
Then the disciples came and asked him, "Why do you speak to them in parables?"
He answered, "To you it has been given to know the secrets of the kingdom of heaven, but to them it has not been given. For to those who have, more will be given, and they will have an abundance; but from those who have nothing, even what they have will be taken away.
"The reason I speak to them in parables is that 'seeing they do not perceive, and hearing they do not listen, nor do they understand.' With them indeed is fulfilled the prophecy of Isaiah that says: 'You will indeed listen, but never understand, and you will indeed look, but never perceive. For this people's heart has grown dull, and their ears are hard of hearing, and they have shut their eyes; so that they might not look with their eyes, and listen with their ears, and understand with their heart and turn-- and I would heal them.'
"But blessed are your eyes, for they see, and your ears, for they hear. Truly I tell you, many prophets and righteous people longed to see what you see, but did not see it, and to hear what you hear, but did not hear it."
Gnosticism is often referred to as the secret teachings. They are not called secret because they are to be kept from others, but because they cannot be understood by everyone. Only those who have achieved a certain level of spiritual growth through experiential knowledge. Gnosticism teaches the inner mysteries to those who can receive them. The apostles were Gnostics and many of the Gnostic texts are the teaching given to the apostles in private. They see with their eyes, and hear with their ears, and they understand.
Mainstream Christianity teaches the outer mysteries or the parables, because they have not yet achieved the spiritual growth necessary to receive the inner teachings. They see but they do not perceive, they hear but do not listen, nor do they understand.
Gnostics believe that the outer mysteries eventually lead to the inner mysteries, and that everyone will eventually achieve gnosis, although it will take more than one lifetime.