Andará is a form of the verb andar, which means 'to walk' or 'to go about'. While similar to caminar (to walk), andar often has a more casual, wandering sense, like 'to be around' or 'to hang out'.
In the song, the singer uses the future tense to express doubt and wonder about the present. She asks, "¿Dónde andará?" and "¿Con quién ahora andará?", meaning 'Where could he be?' and 'Who might he be with now?'. This use of andará beautifully captures her heartache and uncertainty about her lost love.
Peruvian cumbia group Corazón Serrano gives us a song about a promise that was broken, or at least has been left unfulfilled for a very long time. The singer is direct: "Si prometiste un día volver / Yo no sé por qué / Aún no vuelves tú" (If you promised one day to return / I don't know why / You still haven't come back). It's not a recent breakup, either. She makes it clear that "Ya pasaron años" (Years have already passed), and she's still waiting.
This waiting has turned into a total obsession. The song becomes a frantic list of questions as the singer's mind spirals, wondering about her lost love's new life. She asks, "¿Dónde y cómo estará?" (Where and how could he be?), "¿Con quién ahora andará?" (Who is he with now?), and the title question, "¿En qué brazos estará?" (In whose arms could he be?). The constant repetition of these questions shows she just can't move on without answers.