Spam email is similar to junk mail - sent out to hundreds of thousands of email addresses, in the hope that a small per centage wil respond.
Some spam mail is malicious and is used to spread virus that can harm your pc.
If you receive an email from a name you don't recognise, move it to your junk folder and delete it from there. Over time your email provider will come to recognise your junk and it will be blocked.