In terms of accessibility you can get in there on Normal difficulty from Level 98 and upwards, while Heroic / Mythic / Mythic + difficulties await players who own the appropriate item level gear (or can cobble together a private party group). Thicket is the regional dungeon that you'll be led towards during the course of the starting questline for Val’sharah, and from a lore perspective it's the final challenge that concludes Malfurion Stormrage's storyline for this section of the expansion.
