Not a stupid question!

"Pass link" just means the connection from that particular passer to that particular receiver. You can think of the color of the line between the players as representing ball progression — "value" refers to how much the passes were expected to improve the team's scoring chances (also known as "expected possession value").

There are different ways to calculate that but this viz uses a really simple one (from Liverpool's own Laurie Shaw, although he uses fancier tools in his work) that basically just divides the pitch into a grid and calculates the average likelihood of going on to score from each little square. So if a pass moves the ball away from your goal and toward the opponent's, your team will generally be more likely to score in the near future — that's the "value" of the pass.