What does it mean when people are endowed by their Creator with certain unalienable rights

"Endowed by their Creator" means "God-given" rights. In other words, our rights do not come from government or our status as humans beings (those are human rights), our rights are God-given, also known as "natural rights." The Supreme Court generally refers to them as natural rights now. "With certain unalienable rights" refers to the fact that no government, other institution, or person can take anyone's God-given/natural rights away. Nowadays people would say inalienable, unalienable is just archaic. As an example, freedom of religion is a God-given/natural right, we all have the right to believe as we choose. Also in the first amendment, the right to petition our government for a redress of grievances is a human right between the people and the government.