The Yiddish adjective frum (Yiddish: פֿרום;, related to German fromm), meaning "devout" or "pious", is a word meaning committed to the observance of the 613 commandments, or Jewish commandments, specifically of Orthodox Judaism. This appellative is generally, but not only, applied in reference to Orthodox Jews, and used by that group as a self-reference.
The opposite of frum is frei (Yiddish and German "free") meaning someone who is "not religious" and "free" from the yoke of the 613 commandments, or feels "free" to do whatever they feel like doing.
The Yiddish adjective frum can have other specific meanings depending on the context in which it is used. Often the terms frum and frei indicate one's social associations as much as his/her actual religious commitment.
Someone who is extremely frum or devout is known as a frummer.
"Frummer" can also have a negative connotation similar to "Chasid Shoteh" (pious idiot), which is how the Talmud (Sotah 21B) describes a man who sees a woman drowning but refuses to save her for, he says, "it is not proper to look at her, and rescue her." A frummer in that sense is a person displaying a disproportionate emphasis on technical aspects of religion at the expense of worldly or practical concerns.
"Frum" can be used in a negative sense for: "hypocritically pious", "holier-than-thou", "sanctimonious"; or in a positive sense for: "pious", "devout", "God-fearing" and "upright". A combination is sometimes used to describe someone as "frum and ehrlich", which captures all the positive attributes of these words and would roughly mean "upright" or "righteous" (tzadik).
In orthodox communities, the acronym "FFB," meaning "Frum From Birth," is sometimes used to refer to a person who was born into a religiously observant family and has maintained this lifestyle; it contrasts with "BT" which refers to a baal teshuva - a Jew from a secular background who has become religiously observant later in life.