Depends on the context...
In Christianity

Crucifixion of Saint Peter by Caravaggio

Peter's Cross on a Lutheran church
The origin of the symbol comes from the Catholic tradition that Simon Peter was crucified upside down,[1] as told by Origen of Alexandria. The tradition first appears in the "Martyrdom of Peter", a fragmented text found in, but possibly predating, the apocryphal Acts of Peter, which was written no later than 200 A.D. It is believed that Peter requested this form of crucifixion as he felt he was unworthy to be crucified in the same manner that Jesus died. As such, some Catholics use this cross as a symbol of humility and unworthiness in comparison to Jesus.

According to Roman Catholicism, the pope is Peter's successor as Bishop of Rome. Therefore, the Papacy is often represented by symbols that are also used to represent Peter, one example being the Keys of Heaven and another the Petrine Cross.

Anti-Christian imagery
See also: Anti-Christian sentiment
By inverting the primary symbol of Christianity, the upside-down cross has become popular within anti-religion groups.[citation needed]

Many horror films use inverted crosses as part of their imagery and marketing, especially if the film involves demonic themes.

The inverted cross is also a recurring motif in punk rock and heavy metal, where it is embraced as symbol of anti-authoritarianism and defiance (but not necessarily anti-Christian), and is featured in the iconography of punk-themed fashion label Cheap Monday, hip-hop collective Odd Future, worn by fictional bassist Murdoc Niccals of the band Gorillaz, one of the symbols associated with synthwave artist Carpenter Brut, throughout the rock opera American Idiot based on the music of Green Day and worn on a necklace by rapper Lil Uzi Vert. An inverted cross was also commonly worn by artist Lil Peep often in the form of an earring.

Members of the black metal scene often will also paint it onto their foreheads or in between the eyes as a part of their corpse paint (a kind of makeup worn onstage to make one to appear as though they were a dead body). Black metal band Mercyful Fate’s vocalist King Diamond is perhaps the origin of this practice. King Diamond is also well known to use an inverted cross made of bones as a handle for his microphone while performing onstage.

Former Black Sabbath drummer Bill Ward suggested that the use of an inverted cross on the inner gatefold sleeve of their debut album by their label Vertigo may have been a promotional ploy or a misunderstanding of the nature of the group.[2]