Greek: 14 Ὑμεῖς ἐστε τὸ φῶς τοῦ κόσμου. οὐ δύναται πόλις κρυβῆναι ἐπάνω ὄρους κειμένη· 15 οὐδὲ καίουσιν λύχνον καὶ τιθέασιν αὐτὸν ὑπὸ τὸν μόδιον ἀλλ’ ἐπὶ τὴν λυχνίαν, καὶ λάμπει πᾶσιν τοῖς ἐν τῇ οἰκίᾳ. 16 οὕτως λαμψάτω τὸ φῶς ὑμῶν ἔμπροσθεν τῶν ἀνθρώπων, ὅπως ἴδωσιν ὑμῶν τὰ καλὰ ἔργα καὶ δοξάσωσιν τὸν πατέρα ὑμῶν τὸν ἐν τοῖς οὐρανοῖς.
My Translation: 14 You are the light of the universe. A city cannot be hidden when laid on a hill. 15 And people don't burn a candle and put it under a bushel-container, but on a candlestick, and it shines to everything within the house. 16 Even so, shine your light in front of humans, that they may see your good deeds and praise your father who is in the heavens.
KJV: 14 Ye are the light of the world. A city that is set on an hill cannot be hid. 15 Neither do men light a candle, and put it under a bushel, but on a candlestick; and it giveth light unto all that are in the house. 16 Let your light so shine before men, that they may see your good works, and glorify your Father which is in heaven.
Before we begin, let me make a quick note on my translations: I am making these translations partially to be read in conjunction with the more-familiar King James translation. Don't read my translation as being somehow "more accurate" intrinsically, but rather I'm trying to give a sense for the range of meaning that can be expressed by the verse by presenting the two translations together. I'll let you know when the KJV is probably in error and will point out the better reading. If I don't say anything, please assume that while I am giving an accurate translation it's just meant to be read to give some color and breadth to the KJV reading. The King James Version is problematic not because of its errors (though there are errors) but rather because of the age of the English text that makes it ever more obscure as it ages. As a translation, the KJV is remarkably apt at preserving some of the oddities of the underlying Greek grammar (and this is partially why it can be difficult to read at times).
So, first out of the gate is Matthew 5:14-16. This is from what is arguable the most famous sermon in the entire Christian New Testament: Matthew's Sermon on the Mount. A long collection of statements on the Jewish Torah, this sermon presents Jesus as a great Rabbi who is upholding the validity of the Torah even as he presents a viewpoint on it that supersedes and extends it (one reason why some scholars speculate that the historical Jesus might have actually been a Pharisee; I don't agree, but it's an intriguing possibility). The section in question has been famous throughout much of Christianity for centuries (there's even the fun children's song "This Little Light of Mine"). A quick note on the KJV of the text: when it says "Let your light so shine" it's not a nice suggestion that we should allow our lights to shine, but it's a command. The verb is an imperative, Jesus is telling his followers that they must shine. It's not a passive acceptance, but a call to action.
There isn't really much to be said about this scripture that is particular a Mormon point of view, except that as read by most Mormons of Seminary age, the statement is taken as a call for Mormons to let their light shine before the world. But then again, that's pretty much an anecdotal viewpoint based on my own upbringing and childhood experiences. It could be (and probably is) different in some areas of the Church. However, as we'll see in later scriptures, the Seminary scripture mastery scriptures aren't always the best example of ecumenicism anyways.
Why Do I Think This Is Part of Scripture Mastery?
To encourage Mormon youth to be evangelical in their faith through both actions and words. Also, it provides a sense of superiority through membership in the Church by associating Christ's statement of "Ye are the light of the world" to members of the LDS Church.