The truth is simply all the things we know or say which which match the real world.
What you see, feel, touch, taste and smell is true in a basic way. If you see a rock fall, watch it crack in two, hear the noise, and smell some dust then you know the rock has been smashed. You can conclude that it is true that the rock is smashed.
It is possible to be confused about what our senses tell us, but once we put our experiences together and interpret them we can eliminate that confusion and determine what the truth is.
Over time we generalise what we experience into principles. These tend to get more complex and refined over time as more experiences are put together. We might see different rocks fall and sort them into types that will and will not smash in different cirucmstances: granite, chalk etc.
We might learn that a smell in the kitchen is raw fish and conclude we don't like fish. Then we may try a fish dish and discover that it is only bad when it is raw, or rotten. We may meet friends who like the smell of raw fish. We might try sushi. Over time we conclude that fish is good for us, but sometimes we dislike some fishy smells.
Children might notice that what others think about them has consequences, so they lie to make people like them. Over time, they discover this makes life harder rather than easier, and they discover that people dislike them when they see through the lie. This teaches children that honesty is good.
However big and broad the truth is, it is true because of what we see and experience. We remember what is true by remembering the experiences that link the truth we know to the reality we experienced, and we can extend our knowledge by learning about the experiences of others.
The role of epistemology
The branch of philosophy that tells us what is true is called "epistemology". Philosphers have pondered epistemological questions such as "how can know what is really real?" and "we can see what is, but how do we know what ough`t to be?". These issues may seem very dry and technical, and in fact they are. They do also have serious consequences.
For many years mainstream accepted philosophy has taught us that it is not possible to pass judgement on what happens in different cultures. When those cultures impose their traditions in their children then we must decide whether to intervene. Often Western cultures have not intervened in cultural traditions such as male or female circumcission even when the victims live in Western nations.
This is not a dry technical matter for the people involved, the victims, parents or police, but the problem stems from the pages of dusty text books and the classes taught in modern and well respected Universities.