What is meant by a preemptive algorithm?
An arbitrary algorithm can't be preemptive or non-preemptive. Preemption is a mode of multitasking, used by an operating system to control user processes. From the Preemption Wiki page:


  In computing, preemption is the act of temporarily interrupting a task being carried out by a computer system, without requiring its cooperation, and with the intention of resuming the task at a later time.


The opposite of Preemption is Cooperative Multitasking.