Mill conversion is when a factory that used to make one thing starts making something else instead. It's like when you used to draw with crayons, but now you paint with a paintbrush. The factory is like your crayons, and the new thing it makes is like your paintbrush.
When a mill converts, it means that the factory changes what it makes. This can happen for all sorts of reasons. Sometimes the old thing the factory made isn't popular anymore, or it's too expensive to make. Other times, the factory might want to take advantage of new technology or a different market.
For example, a mill that used to make cotton fabric might convert to making polyester fabric instead. The new material might be cheaper, or it might be more in demand. The factory might need to get new machines or retrain its workers to make the new fabric, but in the end, it will be able to produce something that people want to buy.
Mill conversion can be a big change for a factory and its workers. But it can also be a chance to try something new and exciting. Just like how you might be nervous to use a new art tool at first, a factory might be nervous to switch from making one thing to another. But with practice and hard work, they can learn to make something great.