I want to unpack the phrase "good night" further, as it is very important in this poem. As the previous response mentions, "good night" connotes death and dying. "Night" makes sense here—death and dying are usually associated with sleep and darkness, which are also associated with nighttime. It's interesting that Thomas chooses to call this night "good" rather than bad or scary. The word "good" has positive connotations; goodness can be associated with purity and heaven. Though death is certainly dark, like the night, it can also be sweet, comforting, and a good and natural thing.
So, in addition to telling us that we need to live life to its fullest, Thomas also tells us that even though we should "rage" against death and resist it until the last possible second, when our time to die finally comes we should accept death as something not to be feared, but to be embraced. Thomas reinforces this idea when he tells us that "wise men at their end know dark is right."
First, it's necessary to understand the meanings of denotation and connotation.
Denotation refers to a word or phrase's literal meaning; in this case, the denotation of "good night" would be exactly what the phrase literally says: that the night is good in some way.
Conversely, connotation refers to the implied associations or meanings that go along with a word or phrase. In this case, because Thomas' poem is usually taken as a discourse on death, we can assume that the phrase "good night" connotes death and dying. Knowing this distinction between the two terms, it should be relatively straightforward for you to pick out other words in the poem with strong connotations; there are plenty of them!
As far as time and attitude to life in the poem goes, that's a somewhat tricky question. Generally, based Thomas' orders to "rage, rage against the dying of the light," in addition to his desire to "not go gentle into that good night," we can assume that Thomas constructs a defiant attitude toward death and mortality. Rather than simply fading away with old age, Thomas urges us to live to the fullest, to rebel against the frailty of our mortality. As such, he views time in general, the process by which we pass closer to the moment when we must die, as a dynamic entity, something that is meant to be truly lived in, rather than passively existed in.