Can you please explain the first stanza of the poem "Do Not Go Gentle Into That Good Night" by Dylan Thomas?

Do not go gentle into that good night,

Here Thomas begins a straightforward command.  Notice that the syntax of the sentence (a command!) mirrors the meaning--Don't go gently into "that good night."  You probably know that night and death are very much alike, the darkness of closing one's eyes, the fact that the sleeper is in a prone position, the inability to know what is there in sleep or in death.  The speaker in the poem is giving a message which is not a peaceful one about death:  don't accept it, don't glide into it without a fight at the very least!

Old age should burn and rave at close of day;

Many people describe old age as the "golden years" when people slow down and take it easy.  Again, Thomas want to subvert that common image of old people, and of the acceptance of death, by playing with the image of burning and raving.  These are not glowing embers of a dying old fire;  people should not just accept the going out of the fire, but kick it up a notch:  burn.  The verbs are especially important here: full of action, yes, but also senses.  Burning is heat, light, smell, sound.  Raving is action and sound.

Rage, rage against the dying of the light.

Here Thomas is beginning the repetition he'll continue throughout the poem:  a command to defy death, "the night" from line one, the "dying of the light" here.  You'll see that the rest of the poem goes on to describe the different kinds of people and how/why they should rage against the dying of the light.  Notice again the command form in syntax, and choice of verb: "rage."  Do not accept death.