Inner Join means that only rows with matching data in the joined tables will be returned. So if you had an invoices, vendors, and invoicelineitem table set, and you queried it using inner join, you would only get back those rows where your select fields had matching data. An example of the sql using the above scenario is:
select VendorName, i.InvoiceID, InvoiceNumber, InvoiceDate, InvoiceTotal, InvoiceLineItemAmount
from Vendors v
INNER JOIN Invoices i
ON v.VendorID = i.VendorID
INNER JOIN InvoiceLineItems ili
ON i.invoiceid = ili.InvoiceID
Where
InvoiceTotal >= 500
Order By
VendorName, InvoiceTotal desc