The written personality of your business. Voice is the way your company speaks, writes, and represents itself in all marketing efforts.
Here's an analogy.
If you’re an avid reader, you likely name someone as your favorite author because you like the voice in which they write. Great writers are known for their voice: it’s distinctive, original, and in the best cases, unforgettable. The same applies to your company voice.
Why does it matter?
If you sound formal and reserved on your product site but then take a playful tone within your product, you can cause confusion for your users.
How does it apply?
Since user interactions with digital products are almost exclusively online, it’s critical that you maintain a consistent voice and personality to avoid confusion.