Monolid eyes are flat on the surface and don’t have much of a crease, if any. The brow bone is less defined and these eyes are more almond-shaped than round. If you can barely feel any crease on your eyelid, then you probably have monolid eyes. If you are Asian, then you almost certainly have monolid eyes.
This is an attractive eye shape to have. You will want your extension eyelashes to make the most of your features by emphasising your attractive eye shape. But you may want to open up your eyes as focus for this eye shape.
How to achieve it
The style which is most likely to work for your eye shape is the cat or the kitten eye style. Long outer lashes with a sweep at the outer edge of eyes will look attractive.
One view on curls is that, since you are trying to emphasise the look of the eyes, it is best not to use pronounced curls. Another view is that, since Asian eyelashes are often straight and downward pointing, you might like to counteract that effect with ‘L’ shaped curls.