Pivots are points of data within the structure of the animus which allow some unknown person to hack Desmonds memories. They appear after you complete the main story and all of Connor's epilogue missions.
You find them by using points on the map to narrow down where they are located (within the green area)
They appear where other players have placed them and will be "downloaded" to your game every 15 minutes. There are twelve in total and you start with two