Email communication is a very frequently used communication channel. Unfortunately, it is an equally frequently used way to carry out attacks with malicious intentions.
In order to make your messages readable only by the people who are actually allowed to read them, there is a possibility of email encryption. This uses an asymmetric encryption method so that unauthorized persons cannot read the contents of the e-mail.
The e-mail signature serves to end the exchange of the required keys. Furthermore, the signature legitimizes the message and thus confirms that this is the actual e-mail and not a manipulated one.