Like every -ism, also Historicism can be used as an over-simplifying label.
Having said that, the starting point must be Hegel's Philosophy of History; Hegel's philosophy is complex and his Philosophy of History is a relevant part of his system.
A key point is the:
attempt to discover meaning or direction in history. Hegel regards history as an intelligible process moving towards a specific condition — the realization of human freedom. “The question at issue is therefore the ultimate end of mankind, the end which the spirit sets itself in the world”.
Marx and Engels, into The German Ideology as well as in the notes Theses on Feuerbach, contrast their new materialist method with Hegel's idealism:
The materialist doctrine that men are products of circumstances and upbringing, and that, therefore, changed men are products of changed circumstances and changed upbringing, forgets that it is men who change circumstances and that the educator must himself be educated. Hence this doctrine is bound to divide society into two parts, one of which is superior to society.
But, common to Marx-Engels and Hegel is the assumption that history has laws: we can "find reason in history".
Be it idealist or materialist, the science of history can discover the rational "drivers" of historical changes and uses them to explain/understand and predict future developments.
Karl Popper (with an excessive simplification) in his books The Poverty of Historicism and The Open Society and Its Enemies uses historicism referring to:
an approach to the social sciences which assumes that historical prediction is their primary aim, and which assumes that this aim is attainable by discovering the 'rhythms' or the 'patterns', the 'laws' or the 'trends' that underlie the evolution of history". Karl Popper wrote with reference to Hegel's theory of history, which he criticized extensively.
Popper attacks "historicism" and its proponents, among whom (as well as Hegel) he identifies and singles out Plato and Marx. The objection he makes is that historicist positions, by claiming that there is an inevitable and deterministic pattern to history, abrogate the democratic responsibility of each one of us to make our own free contributions to the evolution of society, and hence lead to totalitarianism.
We have to consider also Historical materialism as well as Dialectical materialism; these doctrines fit well with Popper's critique but we can hardly found textual support for them into Marx's original works; see e.g. Amedeo Bordiga, Plechanov, Lenin ans Stalin.