Vidar, the silent one, is the son of Odin and the giantess Grid, or Gridr.
Vidar slays Fenrir in this 1908 drawing by W.G. Collingwood
He is the lord of vengeance, and a god of strength. Some even say he is as strong, or stronger than Thor, his brother.
Odin is constantly preparing for Ragnarok, and one of his ways to prepare was to take care of his death, or at least what happens after it. Odin is fated to meet his fate at Ragnarok.. He is to be killed, and consumed by the wolf spawn of Loki, Fenrir. But, he will not be without his vengeance.. his son, Vidar, moments after his father falls, will come and slay the giant wolf.
All through Vidar's life, he collects leather scraps from shoe makers, other humans, and anywhere he can find it. It is called "The Greatest Boot". With this boot, after his father dies, he will come up to the great wolf, place his foot on the bottom jaw of Fenrir, and rip him asunder. He will grab the top of Fenrir's jaw, and rip him apart. (Kind of like King Kong with the T-Rex). After that, it is said that Vidar does survive Ragnarok and may become one of the "main" gods after the "twilight of the gods".
This is about the extent of what we know about Vidar. He was born so Odin could have revenge after his death, so he went and found a mother for his son. Vidar is not mentioned much elsewhere except for his part in Ragnarok.