The twelve nidanas are a teaching about how life works. It's an idea that says that everything in life is connected and that one thing leads to another. To break it down, nidana means cause and effect. So for each of the twelve nidanas, there is a cause and effect associated with it.
It might help to think of it like playing a game. The twelve nidanas are like the rules of the game. It starts with a spark, which causes something to happen. That something then creates a reaction and so on and so forth.
In life, this means that each experience we have affects us in some way. It changes how we think, how we behave, our feelings, and so forth. This idea is meant to show us that our actions have a ripple effect, and something that may seem small can have far-reaching consequences.