Its usually a sign that your eyes are tired or strained.
For others in can be a nervous twitch as a reaction to stress.
Most of the time its the former.
Adding more light at your workstation (I presume its happening after you've been in front of a computer for hours) and resting your eyes by looking at something else for a while will relieve the tiredness.
Even shifting your eyes to looking at the bulletin board or looking at larger print letters will relieve your eyes.
Also, do some eye exercises to help get the blood flowing in the eye muscles.
If its the latter, you'll have to figure out what's causing the stress and either relieve it or adjust to it.