Tyndarus was the King of Sparta. His wife, Queen Leda, was famous for having sex with a giant swan.
He was Helen's father (or stepfather). He was very worried about Helen's future, since it had been foretold that she would cause a war between the Greeks and Troy that would ruin all Asia and Europe.
When Helen began to attract the attention of suitors, Tyndarus was afraid that all the men vying for her hand would attack him. In other words, if he chose one man to be her husband, the rest would be mortally insulted. Tyndarus sacrificed a horse and made all the suitors swear an oath that they would defend Helen and whoever married her.
This oath, meant to prevent war, ended up causing the Trojan War. The aristocratic Achaeans who attacked Troy were bound by the oath of Tyndarus to help Menelaus retrieve his wife.