When you dither, you're flustered because you can't make up your mind about something. You might dither about what to do if your two best friends invite you to parties on the same night.
The verb dither implies not only that you're having a hard time deciding about something, but that you're upset or agitated about it. You might say to your friend, "Don't just pace around and dither about what to wear! We're going to be late!" In the 17th century, dither meant "quake or tremble," and by the early 1800s it came to mean both "vacillate" and "be anxious."