What the expression means
Words that “fall to the ground” are words that fail to take effect or prove untrue. The verse is not saying that Samuel never misspoke in ordinary conversation. It concerns the words he delivered in his prophetic calling: God did not allow those words to fail.
The expression therefore emphasizes God’s faithfulness rather than Samuel’s independent power. The LORD was with Samuel, gave him His word, and confirmed that word by bringing it to pass.
Samuel is established as a prophet
First Samuel 3 records the LORD calling Samuel while he served under Eli. Samuel then delivered a difficult message concerning Eli’s house. As Samuel grew, all Israel came to know that he was established to be a prophet of the LORD.
The surrounding passage connects three ideas: God’s presence with Samuel, the reliability of Samuel’s prophetic message, and Israel’s recognition of his prophetic office.
A test of genuine prophecy
The law taught Israel that a presumptuous prediction which did not come to pass was not spoken by the LORD. Samuel’s words consistently proved reliable because the LORD had truly called him. First Samuel 3:19 is the narrative confirmation of that calling.