I don't understand the judciary act of 1801, and more specifically, the lines quoted from my book.?

before the act the Supreme Court consisted of six justices. One justice leaving would normally have meant that Jefferson would be able to appoint one in his place. However, the law required that no new justices be appointed until the number had fallen below five (reduction by attrition). So it would be necessary for two justices to leave the bench (reducing it to four) before Jefferson could appoint one (thus increasing the number to five).