In current usage it’s likely referring to the success of groups like ISIS in their recruitment efforts aimed at young dissatisfied men.
They represent a minority of Islam with very radical beliefs and extreme means of spreading their message.
So, if they persuade a young man to join them in their violent actions, people refer to him as radicalised because he is in agreement with some pretty violent things like beheading non-believers.
Also the catch-all term for all of these violent groups is Radical Islamists. This is for clarity to distinguish them from the majority of believers in Islam who have a more balanced viewpoint on how their religion fits into their relationship to the rest of the world.
Radical Islamists try to justify killing others in pursuit of their cause using whatever means they can.
But this doesn’t exclude other groups like the KKK and White Supremacists here in the US. they are also radical and separate from the mainstream.
But because of the recent bombings, beheadings, etc. most of the focus is centered on those who are converted to the extreme and Radical version of Islam.