What is truth?

Is truth subjective or objective reality? Are there differing truths? How do we know what it true? Or, does it even matter? Truth matters only as much as human purpose and progress matters.

Scientists are searching for objective truth about the world (or the world beyond if you are an astrophysicist;). 

Well, it turns out that not everything that is important is objective. In fact, the most important things in life are not objective. How many units of love did your mother have for you? (Even if you were adopted, someone had to carry you uncomfortably for a few months.) Or, how many units of joy does eating your favorite food bring you? I could go on about how subjective things are important. Are humans irrational that what we value most can't be measured though?

Rationality is subjective. As a physician, I am accustomed to having to update my clinical practice based on new findings throwing out prior "rational" expert advice or studies. (For example, look up the flip-flop in peanut exposure recommendations by the American Academy of Pediatrics as one easy-to-understand example regardless of your interests or field). It's not that the medical community in whole is irrational because of a few flawed "expert advice" or "rational studies" ended up being thrown out. We just have updated information on that topic that changes what we consider rational.

Religion has been called irrational, requiring adherents to believe in things that are not objective. However, unless you subscribe to nihilism, you too have some "belief" in something not objective that has intrinsic value. In fact, science, as a system of study, has shown that religious adherents fare better in life than atheist controls, arguably making religion more rational than atheism when taken in whole. Yes, a single belief in isolation may seem irrational, as a splotch of paint may not seem that impressive. But taken in whole, rationality can emerge.

I didn't answer the question posed about what is truth, since philosophers can't simply define truth because they agree that "there is no one concept of 'definition' ”. Religions have a different approach to truth than philosophers, and I encourage you to think about what truth is.