TL;DR
The Preamble is the short opening statement of the Constitution. It says India is a sovereign, socialist, secular, democratic republic. It says we the people have given ourselves this Constitution. It sets the goals: justice, liberty, equality, and fraternity.
What the Preamble says (in simple words)
“We, the People of India” — The Constitution is adopted in the name of the people. Power comes from the people.
“Sovereign” — India is independent. No outside power can tell us what to do.
“Socialist” — The country aims for fair sharing of wealth and resources. The government can make policies to reduce inequality.
“Secular” — The State does not favour any religion. All religions are equal before the law.
“Democratic” — We choose our representatives through elections. The government is accountable to the people.
“Republic” — We have no king or queen. The head of the State is elected or chosen by the people.
“Justice, Liberty, Equality, Fraternity” — These are the goals. Justice (social, economic, political), liberty (of thought and expression), equality (of status and opportunity), and fraternity (unity and dignity of the individual).
Why it matters
Courts use the Preamble to understand the spirit of the Constitution. When we read it, we remember that the Constitution is for us — the people — and that our country stands for justice, liberty, equality, and fraternity.
Related: Meaning of “We, the People,” Justice, Liberty, Equality, and Fraternity Explained, Right to Equality (Articles 14–18).