Someone the speaker loves is gone, and the poem is one long wish to have them back. The speaker never says who this person is, or whether they have died or simply left. That vagueness makes the poem easy to step into, because anyone who has missed a person can fill in the gap.
The speaker lists what is lost: the stories, the jokes, the good advice, the sense that this person always knew what to do. The person also seems to have lived a bigger life, doing things the speaker couldn't and seeing places "near and far." The word "tonight" keeps returning, which suggests a particular gathering where the empty space is felt most sharply.
The mood is quiet and tender, with a weary sadness. The repeated line "I wish you were here" works like a refrain, and the final version adds an honest admission: the speaker knows the wish can't come true, but makes it anyway.