What is Truth?
Truth is the correspondence of a statement with reality. To judge whether a statement is true is to evaluate all relevant and available evidence on the subject and weigh the degree to which the evidence is consistent with the statement.
The evidence relating to any statement will always be incomplete. We can only have greater or lesser confidence in any given statement. The more that confidence is warranted in a statement is the degree to which we can consider it to be a description of objective reality.
Any statement will also necessarily be a simplification of a more complex reality. We will never have perfect understanding of reality due to our limited capacity to understand, but we can continually attempt to improve our understanding.
Why is Truth Important?
Truth is the base on which most other values rely. If we cannot find reliable truth, any justice we attempt to promote is undermined, any acts of mercy will be misdirected, any autonomy exercised will miss its goals, and any attempts at prediction will fail.
How Do We Know What Is True?
I am a skeptic. Specifically, I am a modern scientific skeptic. I try to learn and accept the truth about all subjects to the best of my ability, recognizing that my perceptions are distorted by all sorts of biases.
Modern science has come up with methods to reduce the effects of these biases, and has implemented them to greater or lesser success. The modern scientific enterprise, for all its faults, has had unrivaled success in discovering important truths about the universe and its content.
For scientific processes to function, one fundamental assumption is required: that there exist predictable patterns to reality. I believe that this is the minimum assumption needed to achieve any knowledge by any means. And for science, this is all that is needed. Everything else can be questioned, and science will still work. And if reality does not have these patterns, nothing can be known by any means.
Knowledge is built as a web, with each strand reinforcing the reliability of others. There are no absolute certainties, or foundations, but many things can be and are known to a level of great reliability.
Expertise is to be respected. No one can know everything. People who have devoted their lives to the study of a subject can reasonably be relied on to have much greater understanding of a topic than the average person. This does not make them infallible, nor does it mean that their opinion on subjects outside their expertise is of greater authority. But if experts on a subject come to a broadly shared consensus, non-experts must accept that consensus as the best available position on that topic.