Nonce stands for nonsense number and is a short string of meaningless data. It is used to adjust the difficulty in Proof of Work (PoW) systems. In such cases miners have to search for the right meaningless string such that the block as a whole satisfies the difficulty set by the number of leading zeros required in the hash of the block. Since hashing is a one-way function there is no easy way to find the correct nonce. Instead, miners try random nonces until they find one that works.