The inability to stay focused on a task, or conversely to shift focus away from something that shouldn't be focused on at the moment, is one of the tell tale traits of ADHD. The main cause for this tends to be rooted in the inability for the ADHD brain to prioritize.

Because ADHD brains don't have a properly functional "gate" to block lower priority tasks from taking the forefront, they are naturally going to gravitate towards the thing that is in front of us at that moment.

This can also manifest in the form of getting easily overwhelmed in busy environments such as a gathering with multiple conversations happening at the same time. For the ADHD brain it is so prone to bouncing from subject to subject that it can tend to do the same thing when hearing multiple conversations at once. Someone might be talking directly to the person with ADHD, but while listening to that person, the ADHD brain picks up on a separate conversation happening next to them that is more interesting. It is not long before the person with ADHD is no longer listening at all and is being asked if they are paying attention. It can also manifest as not being able to focus on ANY conversations as there may be too much going on and the ADHD brain ends up helplessly focusing on a word here, a sentence there, and another word somewhere else. This can lead to the person getting overwhelmed and basically shutting down from anxiety.

Not all the focus issues in ADHD brains are lack of focus related however. If someone with ADHD is focused on something interesting to them, it can lead to hyperfocus. This is one of the things commonly labeled as a superpower for people with ADHD. However, it has it's setbacks as well such as susceptibility to time blindness, ignoring other potentially more important tasks, and mental exhaustion.