What is the number after the index in terminal?

Question

After typing for example, firefox & or xpad & in terminal, the echoed output looks like

[number of background programs running] <an unknown number>

What does that unknown number signify?

Except that they are always increasing, they do not seem to follow any particular order, are they some kind of time indicators?

Answer 1

That number is the PID of the application.

PID is a process identifier. Each application run by a user is assigned a number. To learn more about it please refer to Wikipedia