They aren't essential but the buns do like them.
Readigrass is dried grass - and many rabbits like it better than hay so a few handfuls a day can add variety and also get more fibre into them, which is always a good thing.
Plantain is a weed, you may have it growing in your garden. It is one of the plants people often refer on here to 'forage', as people generally go out and pick them from safe areas. Many types of forage have some therapeutic properties as well as being yummy and full of fibre - plantain seems to have a few, such as being anti-inflammatory. Forage and herbs are generally easier on bunnies' tummies than what we traditionally think of as vegetables.
A few other types of forage include: dandelions, clover, bramble leaves, willow twigs, apple/pear sticks, sows thistle, blackthorn and hawthorn.