"What is truth" is one of the hardest questions. In Philosophy, the branch of Epistemology deals with this.
Scientists believe in the Correspondence theory (truth corresponds to facts and reality). This is helpful, but suffers from issues such as
1) It cannot be extrapolated to everything (we are drawing conclusions from limited data e.g. black swans in Australia)
2) Humans cannot differentiate between reality and our perception of reality
3) Science cannot answer what are arguably the most important questions (is the economy doing well, what is love, beauty, etc.)