Sincerity, Good Faith
noun
The idea of sincerity is sincerity. It's also to have good faith in others.
せいい
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
誠意のこもった説明が聞けたので、今回は彼女の言い分を聞いて上げることにしました。
After hearing her sincere explanation, I decided to give her the benefit of the doubt this time around.