AmountCents is the voucher's total, in whole cents — its total debit, which equals its total credit because every voucher balances. It is the size of the entry and carries no direction; the category says which way it went.
Date is when the voucher POSTED — the accounting date the reports window on, which for an imported bank row is the bank's date and not the day it landed here.
Source is where the entry came from: bank_txn for an imported statement line, scan for a receipt or bill read by the scanner, commerce_txn for a sale booked by the store.
Vendor is the counterparty, resolved from whatever the source knew — a bank row's merchant, a scanned bill's supplier. Absent when the source named none.