It means the word is a noun, plural.
But, SlamRN... you were in a dictionary, why wouldn't you look it up there :/
1Neutrino, I was using Linguee. But it doesn't say "Substantiv, Plural" on all plural nouns; or is it a type of plural noun?
I checked your internetpage linguee.com. If linguee says Substantiv, Plural then its already a plural form. All words that I showed Substantiv, Gender or Substantiv, Plural.
Haus -> Substantiv, Neutrum Häuser -> Substantiv, Plural
It means most likely that the word is a so-called plurale tantum, i.e. a word that is used only in the plural. Example: Leute = people, folk; Ferien = holiday(s).
Reynir4536, that's what I thought. I am not sure; I'll have to look into this when I have time. Thanks for your input.