As an adjective (neutral tone on 道), it means 'authentic.' The connection to 'earth + way' is that something truly from its place of origin is genuine.
Don't confuse the adjective 地道 (dìdao, neutral tone, 'authentic') with the noun 地道 (dìdào, fourth tone, 'tunnel/underground passage'). The adjective is far more common in daily conversation.
