It means you’ll have either an incomplete view or something will be in your line of sight because of the position of the seats — like a pole, speakers or the sound board — and you won’t be able to see the entire stage.
The Event Organizer makes these tickets available because for many fans an obstructed view is not a problem. Don’t worry, obstructed view tickets are clearly labeled as such at time of purchase.
But, if you purchased obstructed view tickets in error, contact Fan Support.