SR.N1

"sr.n1" probably stands for "senior noun number 1". A noun is a word that gives a name to a person, place, thing or idea. And "senior" means someone who is older or more experienced than others. So this is probably a way of saying that this particular noun is the most important or the first one in a list of nouns that are senior or important. Think of it like a line of people where the first person is the leader or the most experienced one, "sr.n1" is like that first person in a list of words.