A divine and natural hourglass is set for the salvation of the Kingdom of Judah, tied to the simple developmental milestones of a young child. An existential threat hangs over the kingdom, but a promise is made that this danger will vanish before the child even has time to grow up.
The primary approach among commentators is that this points to a very brief timeframe. It refers to the age when a toddler first begins to speak, or when a child learns by habit to prefer good, sweet foods and reject bad ones. This process typically takes only two to three years [רד״ק, אברבנאל]. Conversely, another perspective suggests that the ability to truly choose between good and evil develops much later, potentially referring to the age of maturity at twenty years old [אבן עזרא].
As for the promise that the land will be abandoned, there are different views regarding which territory is meant. The primary approach among commentators is that it refers to the lands of the enemies, specifically Aram and Israel. The territories belonging to the rival kings, Rezin and Pekah, will be left completely desolate. King Ahaz feels a deep sense of disgust, fear, and impatience toward these rulers [רש״י, מצודת ציון], and the prophecy guarantees their downfall. Historically, this was fulfilled within just a few years when the King of Assyria attacked Damascus and killed Rezin, while Pekah was simultaneously assassinated by Hoshea [רש״י, אברבנאל, ביאור שטיינזלץ]. These enemy territories face destruction directly because of the wickedness of their leaders [מלבי״ם].
On the other hand, a different interpretation suggests that the land being abandoned is actually the land of Judah itself. In this view, the land will be abandoned by the enemy armies currently laying siege to it. These invading kings, who scheme to conquer and rule by force rather than by right, will retreat and never return [רד״ק, מצודת דוד]. Ultimately, this sign is given to King Ahaz to instill a deep trust in God, assuring him that the enemies will fail in their plot to capture Jerusalem and will no longer threaten the kingdom.