It means that the specific lettuce can't be counted, which means that the types of lettuce can be counted. Lettuce, as a noun, means lettuce. When it refers to specific lettuce or lettuce, it is uncountable, and there is no plural form, such as: Washthelettuceandpatitdry; When we talk about lettuce or the plant species of lettuce, it is a countable noun, and the plural form is lettuces, such as: Helikeslettuces, buehedoesn'tlikecabbages, which means that he likes lettuce but he doesn't like cabbage, which means that this person likes lettuce species, not specific lettuce.