Hashing power is the power that your computer or hardware uses to run and solve different hashing algorithms. These algorithms are used for generating new cryptocurrencies and allowing transactions between them. This process is also called mining. If you are mining on NiceHash, you are selling hashing power to the buyers - the people who would like to mine, but they don't have the mining equipment themselves, or they simply want to mine as an investment. Buyers pay to rent the hashing power in bitcoins. Those bitcoins are then paid to the sellers when they forward the hashing power.