Okay kiddo, do you know what "morals" mean? They are like rules that help people decide what is right and wrong. It's like having an inner compass that guides you to do the right thing even when no one is watching.
Now, sometimes people don't follow these rules and don't do the right thing. They may not have a strong inner compass or they may ignore it. This is called lacking "moral fibre". It's like having a weak muscle that doesn't allow you to lift heavy weights.
For example, if someone steals something, they are not following the rules of being honest and respecting other people's property. They lack moral fibre and have made a bad choice that is not nice to others.
So if someone is said to lack moral fibre, it means they are not making good choices and are not being respectful to others. Just like how we have to exercise to make our muscles stronger, we have to learn and practice good values to strengthen our moral fibre.