I would like to buy a laptop and dvd drive is optional dvd drive. What does that mean?
That it is possible to install a Dvd lafufwerk, but none is there. The slot is empty or there's a hard drive / battery in it.
Optionally, not every model contains a DVD drive. There are some with and which without. So you have the option to vote.
That means that you have the option / selection to buy it with or without a DVD drive.
Usually, the laptop has either a drive bay in which this can be installed or it is enclosed with an external DVD drive, which can be connected via USB if necessary.