"Defund the police" has a lot more relevance in the US than in the UK. It also is one of those snappy statements that reduces a complex idea down to something simple and, in this case, rather misleading. There are of course extremists who would take "defund" 100% literally, but more usually it means to reduce the budget and in particular the scope of the policing service.
For example, in the US there is a particular problem with militarisation, when police forces get hold of former military equipment that is at best unnecessary and at worst deadly. Defunding the police would see this access to military equipment and the ability to purchase more of it vastly reduced.
Further, police may handle routine incidents that have nothing to do with crime at all, or at least not a particularly serious crime. This is then a waste of resources, not to mention that the incidents in question may require particular skills that the police either don't have or shouldn't be expected to have. Example:
https://en.wikipedia.org/wiki/Shooting_of_Linden_Cameron -- the simple fact is that the police shouldn't have handled this incident, and certainly not while armed, but part of the reason they did is that an adequate alternative resource isn't so readily available.
The campaign goes beyond this, but there are so many different variations of it that it's impossible to give a complete description.
https://en.wikipedia.org/wiki/Defund_the_police