What s the difference between and in Kotlin

Question

Kotlin is new to me. What makes these two different from one another? and ? below is the code.

Two snippets are provided below: the first use!! for another having an mCurrentDataset? identical variable.

if(!mCurrentDataset!!.load(mDataSetString.get(mCurrentDataSelectionIndex), STORAGE_TYPE.STORAGE_APPRESOURCE))
{
    Log.d("MyActivity","Failed to load data.")
    return false
}

---

if(!mCurrentDataset?.load(mDataSetString.get(mCurrentDataSelectionIndex), STORAGE_TYPE.STORAGE_APPRESOURCE)!!)
{
    Log.d("MyActivity","Failed to load data.")
    return false
}