What does David Hume mean by saying that the human nature is a bundle of perceptions?
David Hume's Bundle Theory is a theory of human nature, or more specifically, of the relationship between perception and the unique "self" that humans believe they possess. Most people consider the mind to be a uniquely perceptual construct, entirely separated from any other person or thing. The mind would therefore exist outside of the physical realm and instead is subject to theories of metaphysics. Hume argued that all human nature, including the theory of self-identity, is the result of a collection of perceptions created by the senses; the mind is its perceptions, and without the perceptions the mind would not exist.
...I may venture to affirm of the rest of mankind, that they are nothing but a bundle or collection of different perceptions, which succeed each other with an inconceivable rapidity, and are in a perpetual flux and movement. Our eyes cannot turn in their sockets without varying our perceptions.
When I turn my reflection on myself, I never can perceive this self without some one or more perceptions; nor can I ever perceive any thing but the perceptions.
(Hume, "Of Personal Identity," anselm.edu)
In other words, the constant flow of perception (sensory stimulus) creates the reactive processes that are termed the mind. The human self -- action, reaction, "original" thought -- is constantly created and recreated due to its properties of perception; those properties are necessary to allow the "mind" or "human nature" the status of objecthood. Hume goes on to state that each person exists in a purely subjective world; his perception of himself is entirely based in the grouping of perceptions that make up himself, and he cannot perceive another person without perceiving the perceptions that make up that person's self-identity. The essential theory is that each person exists in an isolated bubble of perceptions, and does not necessarily perceive the outside world as it objectively exists. Instead, the mind's perception of itself colors the world to make sense inside its own perceptions.