People in Psalm 64 are using their tongues in all sorts of ways.
David is using his to voice his prayer to God (vs. 1). He is calling out with his tongue for the Lord's protection and deliverance (as he so often does!)
Contrast that with how the evildoers are using their tongues. They "whet their tongues like swords" (vs. 3). With their tongues, they aim bitter words like arrows. Their arrows will be met with God's arrow, however, and they will be suddenly wounded (vs. 7). The psalmist concludes about them, "Because of their tongue he will bring them to ruin" (vs. 8).
There is one more mention of the use of the tongue in this psalm. Verse 9 says, "Then everyone will fear; they will tell what God has brought about, and ponder what he has done." The tongue can also be used as an instrument of praise and sharing of God's glory.
The lesson? Be careful how you use your tongue. In this psalm we are given three examples - two positive and one negative. Don't shoot your mouth off like an arrow! Instead, lift your voice up to God in prayer or share good words about Him with your neighbor.