Here two further stages of meditations are mentioned. Stage one meditation is wholesome thoughts about the object : savitarka samadhi.
Stage two meditation is dissolving of wholesome thoughts of the object called nirvitarka samadhi.
Stage there of meditation is all the external thoughts about the objects dissolve and only subtle thoughts remain.
Stage four is dissolving even the subtle thoughts about the object.
Thus when we cross the stages of meditation crossing one layer and another layer we enter to the minutest subtle thought about the object.