The question of imperialism, broadly speaking, is ultimately a way of theorizing why European powers since the 1400s or so have expanded throughout the world and put non-European powers into submission.

Among socialists, imperialism is generally seen as being a force necessary for the creation and existence of capitalism

There are broadly two socialist theories as to why this happens: an explanation by Lenin(the dominant explanation) and an explanation by Kautsky

Lenin's imperialism

Lenin's imperialism had to do with monopoly and finance capital. Finance capital fuels the expansion into colonial territories, whereby colonies have capital invested into them, and the colonies export capital and raw materials to the imperialist core.

Kautsky's imperialism

Kautsky's conception of imperialism is based on "ultra-imperialism", that is, imperialist countries work together to form a cartel to exploit colonies and divide the resources extracted amongst themselves.