JoinForm

JoinForm[type] is a graphics directive that specifies what type of joins should be used to connect segments of lines, tubes, edges, and related primitives.

Examples

Graphics[{AbsoluteThickness[15], JoinForm["Round"], 
  Line[{{0, 0}, {1, 0.5}, {2, 0}}]}]

Please visit the official Wolfram Language Reference for more details.