What is big data ?
extremely large data sets that may be analysed computationally to reveal patterns, trends, and associations, especially relating to human behaviour and interactions. "much IT investment is going towards managing and maintaining big data"
You can help the people here by telling them what exactly you have trouble understanding. This question, as it is, is too generic to give you any more insight than google research.
This is a question you can ask Google.
So, Mehdi Zenasni, you seem to have found your best answer now. I doubt that Google gave you less information than this. Likely a lot more.
After my researchs on it, i still don't know what is big data. Maybe you can help me out