Original: The Countess and Susanna recognize the letter (having seen it previously) and whisper the information on to Figaro, who gets out of this scrape by saying Cherubino gave it to him because it still needed the Count's seal.
Simplified: The Countess and Susanna recognize the letter (they had already seen it) and whisper the information  to Figaro, who gets out of this situation by saying Cherubino gave it to him because it still needed the Count's seal.