“Don’t Repeat Yourself” (DRY). It just means that, when possible, you should avoid writing the same code in multiple places. Repeating code can be bad because:

• If you need to change it, you have to change it in multiple places
• If you forget to change it in one place, you’ll have a bug
• It’s more work to write it over and over again

References

• functional-programming
• object-oriented-programming