Thats the drive speed (read/write). 1x means 11.08 Mbit/s (1.39 MB/s), 18x and 24x means 18/24 times faster as 1x. 18x = 199.44 Mbit/s (24.93 MB/s) and 24x = 265.92 Mbit/s (33.24 MB/s).
They are theoretical speeds, doesn't mean that always will work at those speeds.
In order to write, you should see which is the disc speed.