Original: Mrs. Grose implies that Quint may have been a pederast who preyed on Miles, and that he had a sexual relationship with Miss Jessel, the young and beautiful previous governess. Mrs. Grose hints that Miss Jessell, too, had a relationship to the children that seemed inappropriately close.
Simplified: Mrs. Grose suggests that Quint had a sexual relationship with Miss Jessell, and that they both had sexual relations with the children.