The gap between pure intentions of the heart and the actual words spoken is a constant human challenge. Even when thoughts are completely pure, a person needs the Creator's help to express their prayers properly and avoid stumbling over their words. King David turns to God, asking for divine assistance in controlling his speech.
He first asks God to place a barrier or a guard over his mouth [ביאור שטיינזלץ, מצודת דוד]. Alternatively, this request is not just for restriction, but for preparation—asking God to refine his words so they are pleasant to those who hear them [מאירי]. The purpose of this divine guard is twofold: to prevent harmful or incorrect speech [ביאור שטיינזלץ, מצודת דוד], and to ensure that his words are clear and his prayers are favorably received [רש״י]. The ultimate goal is for the spoken prayer to align perfectly with the inner intentions of the heart [רד״ק, מאירי], ensuring that the carefully prepared order of prayer is not forgotten [אלשיך]. On a deeper spiritual level, this request to guard the mouth serves to correct the spiritual flaw originating from the sin of the First Man, preventing the enemy and the evil inclination from causing a person to fail in their speech [חומת אנך].
The plea then intensifies, asking for an even stricter protection over the lips. Because the lips are positioned further outward than the mouth, they require greater supervision [מלבי״ם]. This is also a renewed plea for divine help to maintain clear and pure speech [אבן עזרא]. The lips are compared to doors, with a specific focus on the upper lip [רש״י]. Just as doors open and close, a person should only open their lips when truly necessary [רד״ק, אבן עזרא, ביאור שטיינזלץ, מאירי]. This double request for protection is necessary because even when speech is well-prepared and pleasant, there remains a constant fear of a sudden slip of the tongue at the very moment the words leave the lips [מאירי, אלשיך].
Another approach understands this final request not as a reference to doors, but to the physical lifting of the lips [מצודת ציון, מצודת דוד, מלבי״ם]. Based on this idea, King David is asking for a level of divine guardianship so extraordinary that the moment he merely lifts his lips to make a request—even before a single word escapes his mouth—God will instantly fulfill his need [מלבי״ם].