As part of a computed retopology operation, you can draw curves to guide the operation. This is completely optional. If you try an automated Retopologize a mesh operation and extract an unsatisfactory mesh, drawing curves can help to construct a more ideal retopology solution.
With a curve network drawn on top of the mesh, you can further specify whether the curves define the basic direction of edge flow (soft constraints), or whether you intend them to plot a specific edge (hard constraints).
To create curves that guide a retopology operation
If you tried an automated retopo operation first, add curves in areas where the computed retopology flow is unsuitable.
For a new operation, all curves drawn on the mesh display in orange. Before you make any specific constraint assignments, all curves are treated as soft constraints.
As a general rule, use hard constraint curves to define the major feature areas of the mesh (where edge creation is required). Use soft constraint curves where you merely want to give some direction on the edge flow.