They have sharpened their tongues like a serpent (comp. Psalms 52:2; Psalms 57:4; Psalms 59:7; Psalms 64:3). Adders' poison is under their lips (comp. Psalms 58:4; Romans 3:15). The meaning is that their tongues inflict wounds which are as painful as poisoned wounds.
The pause-sign, "selah," marks off the first stanza.