TL;DR
The Preamble of the Constitution sets four goals: Justice (social, economic, political), Liberty (of thought, expression, belief, faith, worship), Equality (of status and opportunity), and Fraternity (unity and dignity of the individual). These guide how our laws and government should work.
Justice
Social justice — Fair treatment for all, regardless of caste, religion, or background. The law should not discriminate. Economic justice — Fair sharing of resources and opportunities. The government can make policies to reduce poverty and inequality. Political justice — Every citizen has the right to vote and to take part in democracy. Each person’s vote has equal value.
Liberty
We have freedom of thought, expression, belief, faith, and worship. The Constitution gives us these freedoms as fundamental rights (subject to reasonable limits in the interest of public order, security, and others). Liberty means we can think, speak, and practise our religion without fear, as long as we do not harm others or break the law.
Equality
We are equal before the law. The State cannot discriminate on grounds of religion, race, caste, sex, or place of birth. Equality of opportunity means we should have a fair chance to study, work, and participate in public life. The Constitution allows special provisions for weaker sections (e.g. reservation) to achieve real equality.
Fraternity
Fraternity means brotherhood and unity. We are one people. We must respect each other’s dignity. The Constitution says we should promote harmony and the spirit of common brotherhood. Fraternity supports the idea that the nation is one family.
Related: Preamble of the Constitution (Simple Explanation), Right to Equality (Articles 14–18), Right to Freedom of Religion (Articles 25–28).