Is than what grammatical?

"Than" is not a grammatical term. It's a conjunction, specifically a subordinating conjunction used for comparisons.

Here's how it's used:

* To compare two things or people: "She is taller than her brother."

* To introduce a clause expressing a comparison: "He eats more than I do."

* In comparisons of inequality: "I have more money than you."

"Than" is always followed by a noun, pronoun, or clause that is being compared to the first part of the sentence.