Lemon juice reduces saltiness in a dish by introducing acidity, which balances the salty flavor and makes it less prominent. The sourness distracts your taste buds, effectively masking the salt.
When a dish is too salty, it's because the sodium ions are overwhelming your taste receptors. Adding lemon juice, which contains citric acid, introduces a contrasting sour flavor. This sourness interacts with your taste buds, diverting attention from the saltiness. The acid doesn't actually remove the salt, but it creates a more balanced flavor profile where the salt is less noticeable. Think of it like adjusting the volume on different instruments in an orchestra; you're not removing the salt, but you're turning down its prominence in the overall flavor composition. The acidity also helps to brighten the other flavors in the dish, further contributing to the perception of reduced saltiness. Start with a small amount of lemon juice and taste as you go, adding more until you reach the desired balance.
If you don't have lemon juice, other acids like vinegar (white, apple cider, or balsamic), lime juice, or even a small amount of tomato paste can work similarly to balance the saltiness. Be mindful of the flavor profile of the acid you choose, as it will impact the overall taste of the dish.