Learn French: Cette pizza est trop salée.

In French, adjectives must agree in gender and number with the noun they describe. Because pizza is a feminine noun, the adjective salé (salty) takes an extra e to become salée.

I often hear people use trop to mean 'very'. Does it mean 'very' or 'too' here?

In standard French, trop means 'too' or 'too much' (an excessive amount), which is exactly how it is used here. In casual spoken French, you will often hear trop used as slang for 'very' or 'so' (like c'est trop bon for 'it is so good'), but this sentence is a literal complaint about too much salt.

Why do we use cette instead of ce for 'this'?

Cette is the feminine singular form of the word for 'this' or 'that'. Since pizza is a feminine word, you must use cette. If the noun were masculine, you would use ce (or cet if it started with a vowel).

Is salée related to the French noun for salt?

Yes! The French word for salt is le sel. From that root, we get the verb saler (to salt) and the adjective salé (salty).