Ruby grep - 在数组中搜索字符串的一部分

I'm new to Ruby and a bit confused by the grep command in this block of code. I'm trying to gather all the mailbox names via Net::IMAP and then check them against a mailbox argument. Likely the mailbox name will only include part of the argument. For example, someone may type in "Sent" as the mailbox, but many times the mailbox name will be "INBOX.Sent." I'm new to Ruby and a bit confused by the grep