The Heisenberg Uncertainty Principle states that whenever we measure something on a subatomic level, we disturb what we are measuring. Some physicists and/or philosophers have noted that this is true whenever we observe anything. This means that simply by observing, we affect what we are observing. Under this theory of perception, we don't ever really know an object without the affect of our own observation. This does not necessarily mean we do not know the "truth" of that object. It all depends upon how you define truth. There is the truth of the object by itself (without being observed) and the truth of the object as observed. The truth, as far as perceiving beings are concerned, is a combination of that object in the world combined with the effect of our own perceptions.
This theory of perception (observing the world, and therefore the acquisition of knowledge - epistemology) implies that because our perceptions affect the world we observe, we take part in creating the world that we perceive. Some would argue this means we do not have absolute truth to the external objects in the world. Others would argue that the effects of perception are indispensable and therefore the world we observe is the only obtainable and therefore, only meaningful truth there is.
This combination of passively observing the world and actively affecting it with our observation is a combination of rationalism (reason) and empiricism (knowledge based on experience; in this case, experience of observing the world). In addition to affecting that which we perceive simply by perceiving, it must also be said that when we gain knowledge, we then use that knowledge to structure things we perceive. In other words, the knowledge we gain then becomes a tool which we can use to categorize, map, , deduce, induce, and even predict things we will perceive at a later time. This also supports the idea that observation is not a passive activity. We actively affect and structure that which we observe.
As for the question of truth, the debate is more complicated. One could argue that because of all this interaction between consciousness and environment, we can never know the absolute truth of objects in the world. I would argue that while that may be the case, we can know a functional, meaningful truth of objects in the world by paying close attention to what we observe and how we structure (from prior knowledge/reason) and affect what we observe.