Gloss is generally defined as "an explanatory comment or note added to the text of a book". It is "words of explanation or translation of a foreign or strange word that needs explanation".
Wilcox describes gloss as the following:
Glossing is the practice of writing a morpheme-by-morpheme 'translation' using English words. Glosses indicate what the individual parts of the native word mean. Glosses do not provide a true translation, which would instead use appropriate English ways of saying "The same thing." For example, German Es geht mir gut may be glossed as "It goes to-me good" (the hyphenated gloss "to-me" indicates that it refers to a single word in the original). A true English translation of this expression would be something like, "I'm doing fine." 
Gloss is commonly used in written texts to explain an grammatical structure of signed phrases and sentences. ASL phrases or sentences are glossed in English, but glossing does not mean the same as translating. A gloss of a ASL sentence(s) shows an sequential order in English words that correspond to the ASL words.
ASL curricular texts usually include non-manual markers in glossing. However, gloss notation does not show complex grammatical features, such as facial expression, head-tilting, speed and path of movement, and other subtle details of ASL articulation. Remember one learns ASL in the verbal form, not in the written form; that is, glossing is a written form, not a verbal form.
 Wilcox and Wilcox. "American Sign Language". Handbook of Undergraduate Second Language Education. Edited by Rosenthal, Judith. P 120.