Say that you want to estimate what is the chance that observation 1 fits theory A just by accident (the lower this chance is the higher your confidence can be t

Say that you want to estimate what is the chance that observation 1 fits theory A just by accident (the lower this chance is the higher your confidence can be that theory A is "real"). If theory A has parameters you can adjust, it becomes easier for an observation to fit the theory just by accident (because you can adjust the free parameter). This is a fairly flawed description, but it should be a good enough ELI5.