Google Labs are features that demonstrate and test new functionality projects. You can access Google Labs in both the mail and calendar. They can provide some great additional functionality to boost your workflow.
One important note is that since these are not established Google functionalities, they are not necessarily going to be in the Labs forever. That being said some of my favorite labs have been around for years, so they are still worth testing out.
Google described Google Labs as "a playground where our more adventurous users can play around with prototypes of some of our wild and crazy ideas and offer feedback directly to the engineers who developed them."
To Access Google Labs
From GMail
Settings (Gear Icon) > Settings > Labs Tab
From Calendar
Settings (Gear Icon) > Labs
Recommended Google Labs for Educators
Canned Responses - This is my absolute favorite mail lab, it allows you to make message templates and then insert them into your emails. I use it all the time for default messages that I don't want to keep typing the same thing over and over, I leave blank spaces and then tweak the message.
Mark as Read Button - This allows you to mark messages as read without reading them.
Undo Send - Sent a message too soon? This pops up an undo send message a few seconds after hitting the send button.
Calendar
Hide morning and night - hides a range of times in the morning and at night.
Who's my one-on-one with? - This features displays the attendee's name right on the calendar.
Jump to date - Allows you to quickly navigate to a specific date.