It is explained well on the Wikipedia page of MINIX 3[1]:
> To achieve that, the code running in kernel must be minimal, with the file server, process server, and each device driver running as separate user-mode processes. Each driver is carefully monitored by a part of the system named the reincarnation server[2]. If a driver fails to respond to pings from this server, it is shut down and replaced by a fresh copy of the driver. In a monolithic system, a bug in a driver can easily crash the whole kernel. This is far less likely to occur in MINIX 3.
> To achieve that, the code running in kernel must be minimal, with the file server, process server, and each device driver running as separate user-mode processes. Each driver is carefully monitored by a part of the system named the reincarnation server[2]. If a driver fails to respond to pings from this server, it is shut down and replaced by a fresh copy of the driver. In a monolithic system, a bug in a driver can easily crash the whole kernel. This is far less likely to occur in MINIX 3.
[1] https://en.wikipedia.org/wiki/Minix_3
[2] https://en.wikipedia.org/wiki/Minix_3#Reincarnate_dead_or_si...