What does FIFO mean as a cost basis method?

FIFO means 'first in first out'.... that would mean the first stock you bought is the first stock you sold

that comes up when you bought the same stock on different dates over time 

first shares in (bought) first shares out (sold).  so if you buy on 2 different dates and sell after the second buy. it's the earliest purchase used to determine gain or loss. when it's asking for the cost basis it's asking for the $ amount. TT has no way to determine fifo.