Email works a bit like sending a letter in the post. A letter or parcel is driven, flown or shipped between depots until it reaches the right address; emails bounce through the internet between special computers, called servers, until they reach the email server of the person it is for.
If you send a postcard, anyone—including the postman—can read what’s on the back. The same goes for email: messages are usually sent ‘in the clear’, meaning that anyone with access to a server along the chain could read it as it passes through! This means it is a bad idea to ever send secrets, like bank details, in an email.
It is possible to keep the contents of an email secret by encrypting it—scrambling it using a secret code. This can be hard to set up properly, so if you want someone to know a really big secret, it’s still best to tell them in person!
Message that matters: be careful what you send in the clear