Hope you're well.
This is a phrase that you can use to start a casual email. It means "I hope that you are doing well."
You can use this greeting in emails to friends, coworkers, clients, etc. If you'd like to make it more formal, include the subject "I" at the beginning:
I hope you're well.
You can also write "you are" instead of "you're" for more formality:
I hope you are well.