Lawful Good is one of the Nine Alignments, combining Lawfulness with "Goodness" (good intentions, being oriented towards the Light Side, etc.), both a game mechanic, and an actual concept within the universe of the franchise itself.
Lawful Good characters combine devotion to law and tradition with devotion to "goodness", which means they do not commit acts of aggression without a proper cause (be it self-defense, the defense of other innocents, or an effort to intentionally hijack a criminal's attempt to break the law), they abstain from acts considered morally reprehensible accross most cultures (rape, theft, unprovoked assault, murder, intentional destruction of property), and they seek to uphold the law as much as possible. Lawful Good characters typically believe, that anarchy leads to suffering, and that a strong arm willing to enforce the law without hesitation is the best tool to protect innocents from harm.
However, the laws are not always congruent with what is morally good, even in the eyes of Lawful Good characters - as such, Lawful Good characters may often feel conflicted when hammer of justice often seems to strike down people who "don't deserve it": a thief who was stealing from the rich to feed his starving children, a worker who killed a greedy merchant who ruined his life and openly gloated about it, etc. The fact that the law often seems to protect people that Lawful Good chararacters see as "evil" creates a dilenma that every single Lawful Good character has to resolve internally... or sweep under the rug.
One could say that most people who inhabit a civilization that doesn't follow a darkness-oriented religion are Lawful Good. However, when push comes to show, most people stand for neither the law, neither goodness, only aligning with those values when it suits them. The Church of Titanius places their bets on the Lawful Good alignment through and through, believing that the mark of an educated man is the suppression of primal qualities in favour of civilized and lawful ones. The Knights of the Blood Red Light and other knightly orders all shape their values according to this alignment, emphasizing both strict observance of law and tradition, but also values such as kindness, diligence, charity, compassion for and willingness to help those in need, regardless of whether they belong to one's own group or not.