Custom MX Records lets you enter fully custom MX records to direct your domain's mail to wherever you like. Use this if your has mail service hosted with a different provider and you want emails delivered to your mail accounts there. Your domain's mail provider will give you the MX records to enter here. (They may also give you A or CNAME records to add as well.) Any DreamHost-hosted mail accounts will remain active and stored emails saved, but the panel's Manage Email page will no longer display them or let you configure them. Leave the "...still check my email..." option un-checked to ensure emails are properly delivered to your custom MX records server.
I hope this helps!
Let me know if you have any other questions