What type of phrase is necessary to the meaning of a sentence?

A phrase that is necessary to the meaning of a sentence is known as an "essential" or "restrictive" phrase. Essential phrases provide crucial information that clarifies or specifies the subject or object of the sentence. Without these phrases, the meaning of the sentence would change or become unclear.

For example, in the sentence "The car that was parked outside is mine," the phrase "that was parked outside" is essential because it specifies which car is being referred to. If we remove that phrase, the sentence would simply become "The car is mine," which is less specific and could lead to ambiguity.

In contrast, a "non-essential" or "non-restrictive" phrase adds extra information but isn't necessary for the sentence's overall meaning. These phrases are typically set off by commas. For example: "My car, which is red, is parked outside." Here, "which is red" provides additional information but isn't needed to understand the main point of the sentence.