can you explain the theory of alienation to me in a "explain like I'm five" manner?

Feurerbach's theory of alienation dealt with God. According to Feurerbach, the concept of a God alienates humanity from its true nature. (Check out The German Ideology for Marx's critique of Feurerbach).

Hegel postulated that history progresses through ideological phases called the Geist whereby with each higher phase, spiritual ignorance is reduced and we move towards perfect self-understanding. Marx agreed with Hegel's dialectic method of understanding history but denounced his idealism for materialism. For Marx, spiritual ignorance is alienation and self-understanding is "the realization of his species-being". Thus, Marx's theory of alienation centers around materialism, namely labor.

Marx outlines four types of social alienation in his Economic Manuscripts of 1844:

• Alienation of the worker from the worker — from the product of his labor.
• Alienation of the worker from working — from the act of producing.
• Alienation of the worker from himself, as a producer — from his species-essence.
• Alienation of the worker from other workers.

In sum, workers have no say over what they produce or how it's produced; workers perform menial and repetitive tasks that are psychologically damaging; workers can't impose their own will on their labor; workers exist as a thing, as a commodity due to their relation to production (class); workers are pitted against other workers in a competition which alienates them from their mutual economic interests (this ultimately leads to false consciousness).