Thick Provisioning refers to allocation of storage capacity to virtual disk on the physical disk at the time of virtual disk creation.
There are two types of storage capacity allocation to the virtual disk on the physical disk. One type is on need basis whereas other is the static allocation on the time of virtual disk creation. Thick Provisioning refers to the later one.
For an instance, if a virtual disk of 200 GB is created on a 200 GB physical disk then the virtual disk will consume all the capacity even if there is no data actually residing in the physical disk. Thick Provisioning is not much preferable as it bounds the storage capacity of physical storage but on the other hand, it eliminates the need to allocate the storage capacity on physical disk again and again on need basis.