Tecumseh makes three main points in this speech. All of them are designed to get Native Americans to rise up and fight the Americans. This speech was given as Tecumseh was preparing for what we call Tecumseh’s Rebellion, which started in 1811.
Tecumseh’s first point is that the Americans have eradicated all of the Indians in the east and will do the same to the Indians in the Midwest, where Tecumseh lived. Tecumseh asks rhetorically what has happened to the “once powerful tribes of our race.” He says that they have been destroyed by the Americans.
Tecumseh then goes on to make his second point. He says that the only way to avert this fate is to unite. He decries the idea that any group of Indians can escape destruction by choosing not to fight the Americans. He argues that
The annihilation of our race is at hand unless we unite in one common cause against the common foe.
Finally, Tecumseh argues that war is necessary. He argues that the only way to have peace is to be ready and willing to fight when one is wronged. He says that the people who will have the most peace are those
…who timely prepare to vindicate themselves, and manifest a determined resolution to do themselves right whenever they are wronged.
In other words, he says that people can only have peace if they are always ready to fight resolutely when they are wronged.
As to relevance, that is a matter of personal opinion. I would argue that this speech has little relevance to Native Americans today. It is possible that greater Native American unity would gain them some political victories, but they are so few that they cannot hope to win significant concessions from the rest of America by opposing us. To me, the main relevance of this speech is the idea that one must prepare for war if one wants peace. Today, the US has just signed a pact with the Iranians in which Iran is supposed to take some steps away from their nuclear weapons program in exchange for easing of economic sanctions. American “hawks” could use Tecumseh’s speech to argue that we should be more belligerent towards Iran. They would argue that we are not manifesting “a determined resolution” and that we will not be able to enjoy peace unless we are willing to act more aggressively.