Learn Italian: Questo è tanto cibo. - This is a lot of food.

Because it is acting as an adjective describing cibo (food), which is a masculine singular noun. When words like tanto, molto, and troppo are used to mean 'a lot of' or 'too much of' a specific noun, their endings change to match that noun's gender and number.

Could I say Questo è molto cibo instead?

Yes, absolutely! Tanto and molto are completely interchangeable when they mean 'a lot (of)'. Both are adjectives here that must match the noun they describe.

Why isn't there a word for 'of' between tanto and cibo?

In English, we say 'a lot of food,' but in Italian, tanto works more like the English word 'much.' Just as you would say 'so much food' without an 'of', Italian attaches tanto directly to the noun: tanto cibo.

Why do we start with Questo and not Questa?

Questo (this) is a pronoun here representing the thing you are talking about. Since it refers to a masculine noun (cibo), you use the masculine singular form questo. If you were pointing at a lot of water (l'acqua, feminine), you would say Questa è tanta acqua.