In terms of this quote, the speaker seems to be suggesting that there is no such thing as a complete truth. This would reflect a sense that we only know a limited amount of information, and that what we do know is not absolute. In other words, it seems that the quote reflects that there are levels of knowledge. If this is the case, then we might believe that we know only superficial knowledge...that there are deeper levels of knowledge out there. I do not get the impression that the speaker is accusing the world of lying, but that perhaps we settle for what we know as the most there is to know. If we think about smoking, for instance, we can acknowledge that it is bad for a person. We know it causes cancer, however, this is not the entire truth. After a time researchers realized that smoking was even more deadly for those in the company of smokers. It became evident that smoking is addictive. In light of the quote, then, smoking caused cancer, but there were many other implications ("truths") that were unknown for a long time.
If this is the truth of smoking, how many other things in the realm of what we believe we know are left to be discovered? This quote indicates that we know little; there is much more to learn. Therefore, we know only half of the truth: "half-truths."