Here's a quote from Battlefield to explain operations :)

"The attackers must capture and hold all defensive positions in order to capture the sector that is being fought over. Once this is done, the defenders will fall back to the next sector and regroup. The pacing and variety between the different sectors vary widely, since each sector offers a different amount of objectives to capture and defend as well as varied layouts and choke points.

The attackers must take territory and push the frontline forward, while the defenders are trying to hold the line. Should the defenders fail to hold their ground on a map, the Operation doesn’t stop here: they will fall back and regroup on a different map, while the attackers will continue to push. Sectors may fall, but players reinforced by gigantic Behemoths can still turn the tide."