There are two eyelid problems in Labradors. One is ectropion in which the lower lid tends to droop. A severe example of this can be seen in blood hounds in which the lower lid severely droops down and is red in color. Labradors should have relatively tight lower lids. However, a slight droop can occur, especially when the dog is tired. Generally, ectropion has to be very severe in order to be noted on a CERF form by a veterinary opthalmologist. Keeping the eye clean and use of opthalmic ointment can help with this condition but doesn't cure it.
Entropion is a more serious problem because it involves having the lid and eyelashes turn inward which causes irritation of the eye. If severe enough, rubbing of the eyelashes can cause scarring of the cornea and ulceration. Most dogs with entropion have eyes that continuously weep. Surgery can be done to clip the lid so that it doesn't turn inward against the eye.
If you want more information on either of these conditions, take a look at the Canine Eye Registration Foundation web site at http://www.vmdb.org/cerf.html.