Optical Microscopes use visible light to image a specimen. The lenses in the microscope dictate the amount of magnification shown through the Eyepiece to the viewer. Both the eyepiece and the Objective Lens (the actual refracting lens that magnifies and focuses the image) have a magnification power, which combine to show a total magnification of the specimen. The Objective Lens flips the image while magnifying, so the Eyepiece helps to both enhance magnification and reverse the image again so it can be viewed as is instead of in reverse.
A 10x Eyepiece magnifies 10 times; in other words, an object viewed through the eyepiece, in focus, will seem ten times larger than normal. When coupled with a 10x Objective Lens, the mounted and focused pair of lenses multiply by each other; any lens strength is multiplied by the other lens when viewing. In this case, a 10x Eyepiece and a 10x Objective Lens would combine to show a 100x image, or an image 100 times the actual size.
One Micrometer is 1/1,000th of a millimeter, or 1/1,000,000th of a meter. When viewed through the combined 10x10 lenses in the example above, the 10-micrometer cell would seem to be one millimeter in size, or just large enough to be seen with the naked eye through the eyepiece. A higher magnification lens would be needed to perform any detailed study.