Pid

A pid is the process identification number.  Special pids are swapper (0) and init (1).  

Random pids

OpenBSD (by default) and FreeBSD (must be enabled through a sysctl) choose random pid numbers when a new process is forked, other systems choose the next sequentially available number, and when the maximum pid number is reached the number will wrap around back to the beginning. This means that pids are recycled and on a busy system it may not take long for a new process to take the pid of another process that just ended.