It is part of Technical analysis and it resembles the rate of change in price of an asset.It is taken as the slope of the trendline that tracks the price levels of assets.Sometimes Forex Traders consider the market volume when considering momentum.So if prices are changing rapidly then traders are either buying or selling assets in volumes to push the price change in either direction.A high momentum means that the asset has been overbought and a low momentum means that the asset is oversold.