La guayaba literally translates to "the guava," a tropical fruit. However, in Mexican slang, "la guayaba" can refer to something that is easy, obvious, or a piece of cake.
In the context of the song, the lyrics state, "ya sabían cómo era la guayaba," implying that people knew how things worked or how easy it was for the person being sung about to achieve things or help others. It's an interesting and culturally specific slang term that adds a unique flavor to the lyrics.
This song, "La Guayaba," is a tribute to a beloved family man who passed away. It's a look back at his life, remembering him as the "pilar de familia que siempre ayudó en cada problemas" [pillar of the family who always helped with every problem]. He was someone who always found a solution and worked hard to overcome poverty for his family.
The lyrics describe him as a sincere and true friend, a gentleman with the ladies, and someone with a passion for soccer, even sponsoring teams. He was generous, doing favors without expecting anything in return, and the line "ya sabían cómo era la guayaba" [they already knew how the guava was] seems to be a playful way of saying everyone knew his character. He loved life, was a "gallo que no se rajaba" [a rooster who wouldn't back down], and enjoyed a good game of cards. The song ends with a touching farewell, as he tells his mother and brothers that he's with them and that "el cielo me espera y debo irme" [heaven awaits me and I must go].