Hashing refers to the process of taking different chunks of data and combining them into a shorter piece of info, which in this case known as a hash. A hashing function is what determines the output of the hashing process. It processes input data of any size and returns a fixed length hash.
One of the fundamental features or purpose of a hashing function is that you won’t be able to see original information just by merely looking at the hash. It also makes the prediction of the hashing output impossible simply based on pieces of information that you begin with.
Hash functions are actually part of the algorithm that is used in writing new transactions / adding new blocks onto the blockchain via the mining process. Here’s a video that could further explain the process in a more visual friendly fashion.