Scalping is the way of trading in which we hold our positions for very short time even for just 2 to 3 mints. The target of a scalper is to gain very few pips using very high lot sizes. Suppose you entered a trade with lot size of 5 and after getting just 7 pips you close your trade. This gave you very reasonable profit in a short time. That is scalping.
It gets very risky sometimes so be aware of market movement while scalping.