If you are having trouble understanding your email spam filter, maybe this will help.
Imagine you are a ruler. Like a king or queen; not the measuring kind. People from all over travel to visit you. Some people come to ask your advise. Some people come to tell you about the amount of gold remaining in your treasury. Some come to tell you stories about your relatives. Then, there are some that come to trick you out of your gold. And some that come bringing diseases to your kingdom. And even some that come to you to trick you into giving them the keys to your castle.
In the beginning, it was very easy to tell the good people from the bad, but today, the number of bad people coming is so great that you often send good people away because you didn’t notice them. You decide to hire guards for your castle walls. These guards either let people into the castle when they know the person is good or when they are unsure. But in the event they are certain the person has come for bad reasons, the person is killed immediately without ever telling you. You can now focus on the good people and the kingdom is a much safer place.
The people in this story are your emails. The guards are your spam filter. I hope that helps. If not, at least it was fun to write.