Answer by taking a quote from the following article:

Exodus is an American thrash metal band formed in 1979 in Richmond, California. The band has gone through numerous lineup changes, two extended hiatuses, and the deaths of two former band members. Their current lineup consists of guitarists Gary Holt and Lee Altus, bassist Jack Gibson, drummer Tom Hunting, and lead vocalist Steve "Zetro" Souza. Hunting is one of the original members, and departed from Exodus twice, in 1989 and 2004, but rejoined in 2007.

The band recorded their first album, Bonded by Blood, in the summer of 1984. Audio engineering college student Mark Whitaker, who had attended high school with Paul Baloff, oversaw management and record production for the band at the time. 1984 concert photos from Exodus shows at Aquatic Park's Eastern Front Metal Festival (with Slayer and Suicidal Tendencies) and Ruthie's Inn (with Megadeth and Slayer) were included on the album sleeve inserts. Originally titled A Lesson in Violence, the album was not released until April 1985 amidst creative and business setbacks. Whilst Bonded By Blood is considered a highly influential thrash metal album today, critics have regarded the delay in its release as having hindered the impact the album could have had. As Allmusic reviewer Eduardo Rivadavia would later write in his review for the album: "Had it been released immediately after it was recorded in 1984, Exodus' Bonded by Blood might be regarded today alongside Metallica's Kill 'Em All as one of the landmark albums responsible for launching the thrash metal wave." Exodus promoted the album by going on tour with Venom and Slayer. Four songs from their April 5, 1985 performance at Studio 54 in New York City were filmed and released on home video as Combat Tour Live: The Ultimate Revenge. The band subsequently toured or played selected shows with bands like Exciter, Megadeth, Anthrax, King Diamond, Possessed, D.R.I., Nuclear Assault and Hirax.  Shortly after touring for Bonded by Blood was complete, Paul Baloff was fired from the band allegedly due to his behavior related to alcohol and substance abuse, and was replaced by Steve "Zetro" Souza, who had previously been the lead vocalist for Legacy, an early incarnation of fellow Bay Area thrashers Testament. Baloff went on to form the band Piranha in 1987.  Exodus' line-up remained stable for the recording of their next two albums, and the underground success attained with Bonded by Blood would lead to the group's signing to Sony/Combat Records, who released the band's second effort Pleasures of the Flesh in 1987. The album was engineered by then-unknown Sylvia Massy, who would go on to produce two releases by progressive rock band Tool. In support of Pleasures of the Flesh, Exodus toured with the likes of Anthrax, Celtic Frost, M.O.D., Laaz Rockit and Testament.  Fabulous Disaster, the group's third full-length album, was released in 1989. A music video for the song "The Toxic Waltz" would receive consistent rotation on MTV's Headbangers Ball. While promoting Fabulous Disaster, Exodus embarked on the Headbangers Ball tour with Anthrax and Helloween, which brought the band to a wider audience.  After the success of Fabulous Disaster, Exodus was signed to Capitol Records in 1989, who released the band's fourth album Impact is Imminent the following year. Prior to recording the album, Tom Hunting left the band for personal reasons and was replaced by John Tempesta on drums. In 1991, the band released their first live album, Good Friendly Violent Fun, which was recorded during their 1989 tour.

What does it mean that they were bonded by blood?
The band recorded their first album, Bonded by Blood, in the summer of 1984.