Hjelmslev's theorem is a fancy way of saying that we can find a set of rules to break down any language into its smallest possible parts. Think of it like building with LEGOs - you can take a big LEGO house and break it down into all its individual pieces.
Just like how the thousands of LEGO pieces can build an endless amount of creations, languages are made up of small building blocks called "morphemes". These are the smallest parts of a language that still have meaning - like "un-", "happy", and "-ness" in "unhappiness".
Hjelmslev's theorem helps us identify and categorize these morphemes in a language, which is important for linguists who want to study how language works. By breaking down words into their morphemes, we can understand how different languages relate to each other and how they've evolved over time.
So, in short, Hjelmslev's theorem is a way for us to take apart language and understand it on a deeper level.