What is an AoE Marker? We're glad you've asked, fellow spellcaster!

Let's put it this way: during any of your RPG sessions, did a wizard cast a 20' gas spore in a valley of beasts only to encounter debate about blast impact radius? In such cases, an AoE Marker is the DM's most valuable tool as it can quickly and easily measure spell effect distances on grid and grid-less maps.