Hashing means taking an input string of any length, data or information and giving out an output of a fixed length. In the context of cryptocurrencies like bitcoin, the transactions are taken as input and run through a hashing algorithm (bitcoin uses SHA-256) which gives an output of a fixed length.

Its usually a one-way function where output cannot be used to predict the input. However same output can be generated by using the source data that went in to create the output. Hence, hashing can be used easily to verify authenticity of data by computing its hash value multiple times over but cannot be used to infer the data, that was used to create it.