Left indexing earth radical. The original sense of 填 was filling a pit with dirt, so 土 is the literal material being moved. Anchors the verb in physical filling-up alongside 坑 (pit), 堵 (block), 埋 (bury) - the earth-and-cavity action family.
Right component supplies the sound: zhēn → tián, drifted but related in the dental-coronal series. Same phonetic also yields 镇 'town', 颠 'peak'. The link is opaque to modern ears; mnemonic 'truly fill it in' helps lock the reading in memory.
