The bus factor is how exposed you are if key people, perhaps with critical knowledge or skills, were hit by a bus.
Examples might include the only people able to keep a company running, restore a server from a backup, or someone who held the only key to unlock crypto assets of its customers. As a result, many places are set up with, or required to have by investors or insurers, some redundancy — say two experienced developers with knowledge of how a service works — to minimise the risk of a tragic accident also being a severe problem for a company or its customers.
As a metaphor, the bus factor may be helpful to consider without the need for people to actually be hit by a bus. It may be worth thinking through how you might operate if someone was late, missed their flight, was ill on the day, or simply quit. In operational roles, this will often lead to a minimum of two people for each key position if nothing else to allow one to take a break from time to time.
An alternative framing, with fewer fatalities, is 'what if we all won the lottery?' and so decided to go do something else.