Self Talk refers to the ongoing internal conversation with ourselves, which influences how we feel and behave.
For example, you find yourself in a traffic jam while rushing to work one morning. You self-talk could be pessimistic and you might think, "My whole day is ruined. If I don't get to work on time, I'll never hear the end of it. My boss will think that I'm no good and will surely pass me up for that promotion I've been working all year for." You will then start your day in a bad mood and feel demotivated thinking that there's no point in working hard since you already ruined your chances for a promotion.
On the other hand, you could have a more positive self-talk and think, "I'll probably be no more than ten minutes late. I guess I'll just have to take a quick lunch instead of going out to eat. If I can turn in my report before the end of the day and make sure that it's error-free, I might still have a chance to get that promotion."