«忠» is used for the concept of "loyalty" — a firm commitment to a duty, a matter, or a person.
This Chinese character has the radical «心 heart» at the bottom and the character «中 center» at the top. Therefore, this character makes a strong call to place the heart at the center, or to dedicate the central heart, when attending to a matter or treating a person.
Zengzi (a disciple of Confucius) made three daily reflections, and one of those reflections was: "Have I been loyal in the service I render to others?" (曾子曰：為人謀而不忠乎？); in this expression he used this Chinese character.