This is a very old and simple game from the arcade era and there have been many different versions.
The screen is covered by a rectangle and the player can move along its edges. There are enemies inside the rectangle. The player can start drawing a line from one edge of the rectangle and end it on another point of the rectangle. Then the loop that is formed becomes solid and any enemies within that loop die. But if an enemy touches the player while the loop is unfinished then the player dies.
What is the name of that game?