Imagine you have a bunch of toys and you want to play with them, but you don't want to play with the same toys all the time. So you decide to put some of your toys away in a toy box while you play with other toys. This way you can switch up the toys you play with every once in a while.
Storage effect is kind of like that, but with plants and how they grow. Sometimes, plants will grow better in areas where they don't have to compete with other plants for resources like water or nutrients. So, if some plants are doing better in one area than others, it's like they are putting their toys (their resources) away in a toy box. Then, later on, when the conditions change (like when there's less water or nutrients available), those plants can come back and use the stored resources to grow better than other plants. It's kind of like how you can take out toys from your toy box when you're bored of playing with the same ones again and again.
This can be helpful for plants because it allows them to grow better when conditions get tough, and it can also help different species of plants coexist in the same area without having to fight each other for resources all the time.