A sequence is a self-contained portion of the entire story, usually about 10 to 15 minutes (pages) in length. It has its own tension (not the main tension, but related in some way) and it has its own beginning, middle, and end.
It usually also “belongs” to one particular character - not necessarily always the central protagonist - when its tension relates to that character’s want/need/life more centrally than to the other characters. Many parallel aspects of an overall story are kept alive simultaneously and will appear in sequences where the overall tension is about something else entirely.
For example, the tension of a sequence might be “Will John be able to raise enough money to get into a high stakes poker game tonight so that he can win enough cash to pay back a previous debt to Frank, a hard ass loan shark, who is known to break body parts - lots of them - when he doesn’t get his money?”
A. In the beginning of the sequence, we establish the desire, need, and opportunity for the poker game, plus the lack of funds.
B. In the middle part we track John’s attempts to beg, borrow, even steal sufficient funds.
C. In the end of the sequence, his quest comes to a head when he’s face to face with Micah, a local drug dealer, who loans him the money specifically to pay for a cocaine shipment he wants John to pick up.
This gives the overall shape to the sequence and we know where we are and how things are going by where John’s money quest is in relation to need. But at the same time we are a) finding out about John’s sister, who is dating and transporting drugs for Micah, 2) his brother, who needs John to bail him out of jail after a rampage at a local supermarket, 3) John’s car that is on it’s last leg, and 4) the gun John's best friend gave him for protection... that is actually a starter pistol.
We get information, our story advances, and at the same time, we have a short-term tension to worry about - an outcome to hope for, an opposite outcome to fear.
That tension is what gives shape to the entire 10 - 15 minutes. But the third part doesn’t bring everything to an absolute resolution. Yes, John gets the money. But that just leads him deeper into trouble. So we push forward into the story, and a new tension is created.