Once you decide a transaction will come to an end, the next step is to make a payment.

決済 refers specifically to the settlement of a transaction by means of payment. You'll encounter it in checkout lines at stores with things like キャッシュレス決済 (cashless payment) and スマホ決済 (mobile payment), and you'll also see it during online shopping when you enter your payment information on the 決済画面 (payment screen).

This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well — just look out for 決's reading changing from けつ to けっ.