To "go hand in hand" is a common idiomatic expression in English, which means that it is most often used figuratively instead of literally. Saying that two things "go hand in hand" means that they are found together or that they happen at the same time. For example, someone who enjoys reading might say, "A good book and happiness go hand in hand." Neither a book nor happiness has hands, of course, but the expression is used figuratively to indicate that they are linked.
The wide variety of English idioms using the word "hand" might present some confusion for a non-native speaker. A "hand" may refer to the cards someone holds in his or her hands; a person who works with his hands, as in "a hired hand;" or a particular side of the body, such as "the left-hand side." It also might be used as a verb, as in "to hand it over."
After one knows the meaning of "go hand in hand," however, one can easily see how its figurative meaning came from the literal sense of having hands clasped. If two people are literally hand in hand, one cannot go where the other does not. The phrase has been used in a literal sense since about 1500, with the figurative meaning coming into usage by the 1570s.
This phrase is often followed by "with," as in, "Job satisfaction goes hand in hand with increased productivity." This means that job satisfaction and productivity go together, or to put it another way, that employees who like their jobs work better. The figure of speech emphasizes that one is rarely found without the other and often implies a cause-and-effect relationship.
A secondary meaning of the phrase can refer to people working together for a specific purpose, as in, "Alfred and Mary worked hand in hand to design the roller coaster." Like the primary meaning, this usage emphasizes that two things come together, and it implies intentional cooperation. In this instance, one event does not cause another — Alfred does not cause Mary to work harder — but both work hard simultaneously for the success of the project.