CYL, for Cylinder, refers to the strength of the correction for the astigmatism in the eye. It can be either positive or negative. If there is a CYL value on an eye, there must be an Axis value on that eye.
If "DS" or "SPH" or "spherical" is noted in the CYL space on your prescription, you have no astigmatism in that eye. In that case, enter 0.00 in both the CYL and Axis.
OD-CYL is cylinder correction for your right eye.
OS-CYL is cylinder correction for your left eye.