From Latin optimus best, superlative, good
1. the tendency to expect the best and see the best in all things
2. hopefulness; confidence
3. the doctrine of the ultimate triumph of good over evil
4. the philosophical doctrine that this is the best of all possible worlds (Collins Dictionary)
The word optimism has several definitions, as seen in the extract from Collin Dictionary above, and all have the same meaning: looking on the bright side of life; expecting good from life, circumstances, people; being hopeful of good outcomes and cheerful; good will prevail.
So someone who has optimism or is optimistic, like a poet for example, is someone who is a cheerful person who sees the happy or hopeful side of events and people. This is someone who has faith in a better tomorrow and a redeemable today. To put it in the antithesis, this is not someone who is easily depressed or who readily finds fault with others or who is antagonistic and negative. Such a person as this one who is antithetical to the optimist is called a pessimist.
Using our example, a poet who expresses optimism will find solutions to the problems they pose; will write of nature as a friendly and inspiring force rather than as a ravaging force that cannot be controlled; will write of relationships that give love and joy and goodness rather than that oppress and offer anger. Poets being what they are--users of symbolism, metaphor and imagery--their optimism may not be as readily apparent as the optimism in, for example, an optimistically lighthearted Shirley Temple movie.