The gemara on daf 8a quotes Rava who says that if a room is completely open so the sun lights up the entire room then you can check by the sunlight. Rashi says that this means that you don't need to use a candle at night but you can just check it during the day. The gemara on daf 4a says that we should check at night because that's when people are around and candlelight is good for bedika.
So the Tzlach asks how can our gemara say you can check by the sunlight if the light is good? You still need to do the bedika at night because that's when people are around!
He suggests that the gemara isn't giving two different reasons why you need to do the bedika the night of the 14th. First of all the gemara says you should do it when people are around which would exclude a regular workday. The gemara then answers the question it doesn't ask - everyone is around on the 14th in the morning so why can't you do the bedika then? So the gemara answers that the light from candle is better so we'd prefer to do it at night. Now the gemara is saying that's not always true. If it's a room which is completely open then the sunlight is just as good (or better) so you can check during the day on the 14th.