The poem contrasts material needs with emotional ones. In Maslow's hierarchy of needs, after basic needs such as food, water, and safety are met, the individual feels psychological needs like belongingness and love. The speaker here has a home, water, and bread, i.e., all the material necessities. But her emotional needs are still unmet. And following Maslow, it is precisely because her material needs are fulfilled that she longs for something more meaningful in her life.
So even though she herself has a home (she's lying down somewhere), her soul does not seem to have one. Also, she has bread and water, but the water is thirsty and the bread is stone, because they don't meet her emotional needs. In fact, they simply make her more aware of how much is lacking in her life. What she's lacking (as @Randal'Thor's answer says, is companionship: "nobody / Can make it out here alone".
Another aspect is that the soul's home is, of course, the body. The poem goes on to talk about people whose material needs are more than adequately fulfilled ("millionaires / With money they can't use") but whose emotional lives are suffering:
Their wives run round like banshees
Their children sing the blues.
Angelou says that a surfeit of materialism has led to spiritual death: our souls no longer have homes. Essentially, we have forgone emotional connections just to focus on material success. The poem begins with just the speaker's loneliness, but extends this to the state of the world generally, where economic gains are valorized while spiritual, natural, or human values are not. Where water is not thirsty could also refer to the impact of economic progress on the environment. As we prioritize economic growth, we overlook the fact that waterways become less and less able to sustain life. That makes water itself thirsty.
The poem's paradoxical claim that water can be thirsty lends itself to exploration of that claim's meaning, and that meaning in turn can be several layers deep. All these are explorations of possible meanings of the line, not a definitive statement about what the line means. Pinning down one meaning for the line would reduce the power of the line as poetry.
