The violet area is the limits of where the tiny white guy in the middle can move to.
While the game is turn-based, the map does not make use of grids. Basically I just used my idea from Death Zone Zero, which in turn, got its idea from RTS games in general. If you've played tabletop wargames, things work that way.
Movement is calculated as a stamina cost per meter, not a predefined value. He has, in this example, 100 stamina points, and movement is 2 stamina points per meter.
Other actions like attacking also consumes stamina, so the player has to be mindful of deciding when to conserve stamina for movement or actions. Basically the same with Action Points of XCOM games.
Furthermore, climbing upwards has a higher stamina cost of 10 stamina points per meter, which accounts for the irregular shape of the movement range.
The currently selected destination is shown with the X-mark on the ground there, with the distance to that shown at the top left, together with the total stamina cost to move there. You can see the stamina cost is roughly twice the distance. This is correct since again, I've set it to be 2 stamina points per meter. The disparity is from the fact that the terrain is bumpy, and since climbing upwards is more costly, the destination's stamina cost reflects this.