What is the Time War and why is it important to the Doctor's story?

The Time War is a significant event in the history of the Doctor, a Time Lord from the planet Gallifrey. It is a conflict between the Time Lords and their enemies, the Daleks, that took place in the final days of the Last Great Time War. The war was fought across time and space, and it resulted in the apparent destruction of both the Time Lords and the Daleks. The Doctor was forced to end the war by using the Moment, a weapon of mass destruction that could destroy both the Time Lords and the Daleks, as well as all of time and space.

The Time War has had a profound impact on the Doctor's story. It led to the destruction of his entire race and the loss of his home planet, leaving him as the last of the Time Lords. The trauma of the war also affected the Doctor's personality and behavior, making him more brooding and tormented. The war has been mentioned many times throughout the series, and it has been shown in flashbacks and in the form of the War Doctor, a previously unseen incarnation of the Doctor who fought in the war.

Overall, the Time War is a pivotal moment in the Doctor's story, and it has shaped his character and motivations in many ways. It has also been a significant plot point in the series, serving as a catalyst for many of the Doctor's adventures and conflicts.