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2mm is the focal length of the eyepiece. The long eye relief (20mm for this eyepiece as stated on the web site) makes this eyepiece suitable for eyeglass wearers but can be used by non-eyeglass wearers. Eye relief is the distance from the eye lens where you place your eye to see the full field of the eyepiece.
2mm is extremely short and will give you very high powers on all but the shortest telescopes and then these 'scopes will need to be high quality apochromatic (APO) refractors to give you a good image. If you are using a high quality APO then you will do better with a high quality 2-4mm zoom eyepiece such as the Nagler Zoom.
What telescope do you have since its type and focal length will determine the best size eyepieces that you can use?
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