ELI5: Explain Like I'm 5

Convention over configuration

Convention over configuration is a way of making things easier to do by having a set of standards that everyone can follow. It means that something is already set up and all you have to do is follow it. For example, if you were making a cake, you would expect to find the ingredients and instructions in the box the cake mix comes in. You don't have to pick out each ingredient or figure out what steps to take to make the cake - it's all already set up for you. Convention over configuration makes things easier because you don't have to think about what to do. You just have to do what is already written.
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