This is part 8 of the series of post around Agile.
Sprint Team would do well to note down any dependencies between User Stories
while estimating and any dependencies between User Stories and external factors
should there be any.
A dependency is a task that is required to be completed before another task can either be started or finished. An example of a task that must be completed before another task starts, a door must be opened before anything can pass through it. An example of a task that must be started before another task can start could be that you need to start charging your phone before you can use it, although the task of charging your phone doesn't need to be complete in order for you to use it.
Often there are multiple tasks that need to either be started or completed before successor tasks can start or finish. This as you can imagine can get very complicated and may require a system to manage and keep on top of.
There are a number of techniques to identify dependencies including work breakdown structures, but this post is to merely suggest that using an agile methodology still means that dependencies require attention.