An isolated CPU incurs reduced jitter and reduced interruptions by the kernel. This is achived by clearing the CPU from user-space processes, movable kernel threads, interruption handlers, kernel timers, etc. The only fixed source of interruptions is the 1Hz tick maintained by the kernel to keep CPU usage statistics. Otherwise, the incurred jitter and interruptions, if any, depend on the kernel services used by the thread running on the isolated CPU. Threads that run a busy loop without doing system calls, such as user-space drivers that access the hardware directly, are only expected to be interrupted once a second by the 1Hz tick.
A housekeeping CPU is the opposite of an isolated CPU. Housekeeping CPUs run all daemons, shell processes, kernel threads, interruption handlers and work that can be dispatched from isolated CPUs such as disk I/O, RCU work, timers, etc.