In salesforce mass email option is used to Send an email to a group of contacts, leads, person accounts, or coworkers all at the same time.
Step 1 : Depending on who the recipients of your mass email are, do one of the following.
For contacts or person accounts
On the Contacts or Cases tab, in the Tools section at the bottom of the page, click Mass Email Contacts.
If you use person accounts, the link also appears on the Accounts tab.
On the Leads tab, in the Tools section at the bottom of the page, click Mass Email Leads.
For coworkers (other Salesforce users)
If you have the Manage Users permission, from Setup, enter Mass Email Users in the Quick Find box, and then select Mass Email Users.
Step 2 : Select a list view of recipients, and then click Go!.
Step 3 : All recipients in the list view are selected by default. To remove recipients, deselect them.
Step 4 : Click Next.
Step 5 : Choose an email template.
Step 6 : To view the templates, click the Preview links.
Step 7 : Click Next.
Step 8 : On the confirmation page, select the processing options to use. The mass email name that you specify appears in the queue and status messages that you receive from Salesforce, not in the email that you’re sending.
Step 9 : In Delivery Options, send the email, or choose a date and time zone for sending the email in the future.
Step 10 : Click Send, and then click Finished.
This information is very useful to learn about Mass email option.