SMTP credentials are used by the app to send outgoing emails from your account, including emailing purchase orders to your vendors and notifying you of any worthy developments (i.e. errors!). If you don't already use an outgoing email service you're happy with, we recommend https://sendgrid.com, http://gmail.com, or https://mailgun.com.
To setup or modify your SMTP credentials, go to Settings>>General Settings>>SMTP (tab in "Configuration" box). For in depth info on SMTP setup, please checkout the SMTP section in the Settings chapter.