A bit is a section of public speaking material that is so related that it makes it easy for you to memorize. Each point flows naturally from one to the next so you can deliver the information without notes (if you know your material).
Until I learned about bits, I never thought I could be that great at speaking because I'm not great at memorizing long talks. I discovered that no one memorizes long talks. They have a mental or written outline consisting of key words that trigger the individual bit in their minds. Pros use this concept to be able to deliver long presentations to the public without the use of notes.
Becoming less dependent on notes has several advantages. When you stand before a group and speak without using notes your credibility automatically rises. The audience thinks, 'Wow! This person really knows the material.' Since you won't be tied to a lectern or forced to hold notes, you can get physically closer to the audience, or actually enter the audience on occasion. The closer you are to them, the better you will connect. When you leave the script at home you can speak naturally to the audience rather than read to them. You will also be more confident because you no longer have to worry about your notes getting lost.
Using bits has another big advantage. We are busy people. It's tough to find a spare hour or day to practice a full public speaking presentation. Bits can be practiced when you have a few minutes here and there. You will be more likely to practice your material (and we all need practice) if you can practice a three or five-minute chunk rather than the whole presentation.