Liberals have long been in favor of police and prison reform, whereas leftists consider police and prison reform to be good but want to go further and are in favor of police and prison abolition. Both the reform movements and abolition movements have been active for decades, but the reform movement is more pallateable to the liberal, neoliberal, and conservative establishment and therefore got more attention. Reformers and abolitionists often both work together on several protests, policies, projects, etc... but their goals are different and sometimes contradictory with one another.
Black Lives Matters has many chapters, some favor reform, others abolition (though abolition seems to be growing right now), but reform and abolition are not only racial issues, social democrats have often been in favor of reform or abolition, socialists (while some are more comfortable with reform) have often favored abolition (especially libertarian socialists, like anarcho-communists), the queer movement historically was in favor of abolition as police and prisons were used to criminalize and police the bodies and lives of the LGBTQIA+ (though the liberal queer movement wants to focus more on marriage to the exclusion of the material aspects of LBQTQIA+ marginalization and oppression), disabled people have long since known the dangers of the prison-industrial-complex and fought against a version of it known as disability institutionalization and mental illness institutionalization, many people have been supporting some aspects of abolition for a while in the form of opposing immigrant detention and ICE as well as the support for easier legal immigration (and citizenship process) or even open borders, etc...
The defund the police movement/tactic was originally abolitionist, who sought to reduce the power of the police and the police state, and redirecting that funding towards social programs to reduce poverty (which will reduce crime), to provide community education (to reduce social attitudes that lead to crime and to help better communities so they don't need to rely on crime), to support diversion programs that keep people out of prisons and in their communities for rehabilitation and the process of restorative and transformative justice, and so on. Reformers realized the potential in that tactic, seeing as their previous reforms did not work and adopted it for themselves as it does not have to necessitate the abolition of the police. As a result you have different versions of the Defund the Police movement, the original version which is most popular among actual leftists including a sizeable portion of the Black Lives Matter movement looks at defunding the police as step one, whereas the versions being pushed by the media are the more tame reformist versions. Vox did an article discussing the various versions of what defunding the police means, but I don't like the article very much because it only covers reformists versions, with one exception and it tried to tone down the policy positions of the abolitionist meaning to sound more ambiguous.
