A gloss is a way to show how a sentence is broken up into separate parts and what each part means. Most linguists use a standard system for this called Leipzig glossing.
The sentence “I see dogs” could be glossed as “1S.NOM see dog-PL” in the Leipzig glossing system. 1S.NOM means the nominative (subject) version of the first person singular pronoun. dog-PL means it’s the word dog with a bit added to it to show plural. You can see the entire Leipzig glossing system here: https://www.eva.mpg.de/lingua/pdf/Glossing-Rules.pdf
