Imagine you and your friend both have a secret code that you use to talk to each other. However, your friend speaks a different language than you do, so you have to translate the code to understand what they mean.
Now, imagine that you and your friend are both scientists studying a new language spoken by an alien species. You don't know what the words mean yet, so you create a code to represent the words. However, you and your friend have different ideas about what the code should be, so you end up with different translations of the same words.
This is what indeterminacy of translation means - even if you have a code or language to represent something, different people might interpret it in different ways. It's like trying to solve a puzzle when everyone has a different idea of what the picture is supposed to be.