Learn Italian: Ieri la folla era arrabbiatissima. - The crowd was very angry yesterday.

La folla (the crowd) is a collective noun. In Italian, collective nouns take strict singular agreement, even though they represent a group of people. So we use the third-person singular era, never the plural erano.

Why does the adjective end in -issima instead of -issimi if there are many angry people?

Just like the verb, the adjective must agree grammatically with the noun la folla, which is feminine singular. Therefore, the absolute superlative becomes arrabbiatissima to match the feminine singular noun, regardless of how many people are actually in the crowd.

Why use the imperfect era instead of the passato prossimo è stata for something that happened yesterday?

The imperfect is used here because it describes a continuing state, condition, or emotion in the past. Even though we specify ieri (yesterday), era sets the scene of how the crowd was feeling, rather than treating their anger as a single, finished action.

What exactly does the -issima suffix do to the word arrabbiata?

Adding the suffix -issimo/a/i/e creates the absolute superlative, which is a built-in way to say 'very' or 'extremely'. Since arrabbiata means 'angry', arrabbiatissima means 'extremely angry'.