The poem "Uphill" is a metaphor representing a difficult, arduous journey. Indeed, the phrase "an uphill struggle" is a common idiom even today. Throughout the poem, the speaker enquires about the journey and is told that there will be shelter when she needs it, "other wayfarers" that will welcome her when she sees them, and a resting place at the end, in the form of a bed, for her and for all those who make the same journey.
If we interpret the journey itself as a metaphor, then the shelter, the wayfarers, and the bed are also all parts of the metaphor. They all represent how she will not be alone on the journey, and how she will be cared for and given a reward at the end. In other words, this will be a difficult journey, "up-hill all the way," but it will be worth it in the end.
One interpretation of this poem is that it serves as a metaphor for one's journey to God. Rossetti was a devout Christian, and the poem can be read as a conversation between her and God. Indeed, structurally, each stanza is a back-and-forth dialogue between the speaker who enquires about the journey and the voice who replies with reassurances. The fact that the voice always replies indicates that God will always be there for the speaker, accompanying her all the way on her journey. This touches upon the main theme of the poem, which is the benevolence of God.
If we break down the metaphor, then the speaker's journey to God is an uphill struggle because it requires patience, humility, asceticism, and devotion. The fellow wayfarers who she will meet on the way are fellow Christians. The bed she is promised at the end of the journey is the respite she shall receive in heaven.