A hash is something like a digital fingerprint for data. Imagine you have a large text, for example, a book, and you want to store it somewhere, but not in its entirety, rather in a short version, so you can quickly check if it has changed. This is where the hash function comes into play. A hash function is a mathematical algorithm that takes large amounts of data and transforms them into a short set of characters of fixed length, known as a hash value.
A hash value is the result of the hash function's work. For example, if you "pass" the text "Hello, world!" through a hash function, you will get a short, incomprehensible sequence of characters like "5d41402abc4b2a76b9719d911017c592". This sequence is unique for each input text (if you change even one letter in the input data, the result will change dramatically). So, if you get the same hash value, it means the data has not changed.
This mysterious world of data is often used to check the integrity of information, say, when you download files from the Internet and want to ensure they have not been altered. So, the hash function is a kind of "honesty check" that helps us make sure that the data is all right.
