ISO 15919 is a set of rules that tell us how to write Indian languages using the English alphabet. This is helpful because not everyone knows how to read or write all of the different languages in India, but many people can understand English.
Let's take an example of Hindi language. In Hindi, we have many different letters that we use to make words. When we want to write these words in English, we need some rules to follow so that everyone can understand what we're saying.
ISO 15919 tells us which English letters to use for each Hindi letter. For example, instead of writing the letter "भ" in Hindi, we would write "bh" using the English alphabet. The rules tell us what to do with all of the different letters in the Hindi language.
These rules are important because they help us to communicate with people who speak different languages. When we follow the rules, people can read what we write and understand what we mean.