So, now you've got the cut-n-pastes from Wikepedia or some such, telling you that it's an "organic pineapple known as historismus...." All clear now? No! Actually, this not a very good article. It's far to long on big-name philosophers, like Hegel, and sadly short of actual working historians who did the empirical work to establish this theory.
Historicism is what is known as a world-view. It's a way of looking at everthing in the world as essentially the product of historical development. The precise nature of that development is open to a variety of views and is still hotly debated. However, what all historicists can ultimately agree upon is the importance of "development". That is to grasp that every aspect of history must be encountered "on the move", as it were, and in process of change.
There have been many, many historians over the centuries who have maintained the importance of the past in the formation of the present. Who could argue? However, it was not until an obscure 18th Century writer called Herder published a book called the Idea for a Universal History (Ideen) that historicism (as yet not called that) was born. Importantly, Herder maintained that every culture has its own in-built passage of development that cannot be altered by externalities. That's why the Nazis loved him, by the way. He was the founding father of multi-culturalism, ironically. However, Herder was short on theory and long on loving description and that has always been the halllmark of historicism.
More on this later.