explain like i'm five

I take it that you're talking about trade price?
It means that the person you're trading wants something worth 1,5 TF2 case keys (so it would be 1 TF2 key + 1 Tour of Duty (ToD) in most cases). 1 ToD is generally worth ½ key.

key decimals can also be traded with tf2 metal, cards or gems.

1,5 keys means 1 key and a half. The half could be considered a tour of duty ticket (tod) or a capsule key, although a capsule key's value and a tod's value is usually lower than half a key, so many traders ask for a bit more, like refined metal or cards.

one full key and half of another one