What does Playmaker mean when he says "Arrowheads confirmed!"?
The number of arrow heads on a Link Monster corresponds to its link number, so I've always taken this as a declaration that Playmaker is about to perform a Link Summon and that he has the correct materials to do so. Other characters use the phrase as well. From what I've seen of VRAINS, Playmaker sometimes follows the "arrowheads confirmed" statement by saying what link materials he needs.

It is like in Yu-Gi-Oh! Zexal and onwards, how some characters when performing an Xyz Summon will say "I'll overlay level X monster1 and monster2 to build the overlay network". It could just be for our clarification and the duellists.