In Existentialism is a Humanism, Sartre argues that humankind's existence precedes its essence. What does this mean, why does Sartre believe it is true, and what effects would it ultimately have upon morality if it were true, especially related to at least three emotions?

Jean-Paul Sartre was a French writer, philosopher, and political activist who was widely influential in the burgeoning existentialist movement of the twentieth-century. Existentialists believe that thoughtful reasoning, thinking, and questioning have its origins in the human individual. As such, it is that individual rather than the society that is most responsible for infusing life with meaning.
In 1946, Sartre published his book Existentialism is a Humanism, which posited that our "existence" preceded our "essence." We are real living beings who create our own meaning, our own nature and our own identity through choice. In other words, our tangible and immutable characteristics as human beings (existence) allow us to dictate our nature and the traits that make us unique (essence).
Our essence as humans might be that we walk, speak, think and love. However, not all human beings who exist have two legs or a working tongue. Some use their brain capacity more than others and some may love more superficially or more selflessly than others. Sartre is saying that once we exist, we can create our own essence through our consciousness.
This assertion was a profound 180-degree flip of the prevailing wisdom that essence, not existence, came first. Traditional views on the subject held that our essence was our soul or some common intangible that gave us a pre-established purpose and that existence followed from that. Sartre was well known to be an atheist, and as such, his lack of faith in a supreme moral authority grounded this belief that essentially man is God and the ultimate arbiter of his destiny through absolute freedom of choice.
Without a supreme moral authority such as a God in the picture, all morality becomes relativistic in Sartre's world. Man exists first and then can create his own nature as he sees fit, including his own moral worldview, which leaves open plenty of room for questions of what is truly right and wrong.
Moreover, Sartre calls anguish, abandonment and despair three emotions that are an "inescapable part of the human condition." This statement appears to contradict his notion that humans are completely free to make choices consciously about their nature and what they want their essence to be. But it reinforces his ideology that there is no perfect being (one whose image we've been made in) that is free of these things. As such, he seems to indicate that these traits are more a function of our existence than our essence, since they are as inescapable as our very being.
https://philosophynow.org/issues/15/A_students_guide_to_Jean-Paul_Sartres_Existentialism_and_Humanism