This line means, “And may the earth tremble in its centers.” The language is intentionally grand and dramatic. It imagines the force of Mexico’s response as so powerful that the ground itself shakes.

Retiemble comes from temblar, “to tremble,” with re- adding intensity. En sus centros is poetic and not a phrase most people would use in normal conversation; it suggests the deep inner parts or foundations of the earth. The line is not a literal geography lesson. It is patriotic hyperbole: the nation responds with earth-shaking force.