The brain has two distinct states when learning something new, focused mode and diffused mode. Focused mode is exactly that, short spans of concentration and focus, where you block all distractions and focus only on the task at hand.
It's useful when taking in the detail of something new or when you are looking at something that has a lot of new concepts.
It's important to have regular breaks as it is exhausting for the brain to concentrate for too long. That's why the Pomodoro technique is useful.
Here are some ways to improve focus.
However, it's important to allow time for the diffused mode to take over as well, otherwise you risk tunnel vision.