An Email Blast is an electronic mailing, sent all at once to a large mailing list. This is commonly used by marketers who want to send email advertisements or promotions to their list of customers.
Email blast usually is done with the help of special software or through an email service provider.
Email blasts can be a good way to advertise to customers, but the key is that the people on your list must have "opted-in", meaning they signed up to receive your messages.
Sending advertisements via email to purchased lists or random people is considered spamming