I want to understand how bitcoin price fluctuation. ??
As others said, buyers and seller, but if you want to model it mathematically, in Quantitative world, an extremely simple definition would be: the price of an instrument follows Brownian motion with a drift.
But in reality, beside the drift added to the noise(Wiener process), you need to add other factors like a shock process, to simulate price spikes(up or down), regime shifting(to simulate dynamic volatility) and so on.
To make a long story short: if you want to understand what makes the price fluctuate AND profit from it, you will need to decompose the price structures(Fast Fourier Transform for example) and try to find a model which generates that signal and then cumulate all the signals. The problem will be to identify the correct market regime in advance to properly set models parameters.
Not impossible but you need to know what you are doing
