What does the Latin word magna mean?

magna is an adjective meaning "great." In Latin adjectives, like nouns, come in number and gender and also decline. Magna is the feminine form of the word.

Magnus, magna, magnum. Masculine, feminine, and neuter. They do this so they can agree, just like English, with the noun they modify. So if used to modify a masculine noun, you use the masculine form of the adjective:

magnus rex = great king. because rex is a masculine Latin noun. Or:

magna navis = great ship because navis is a feminine noun, and so forth.

Now you know when you see Magna Carter it means Great Charter.