The practice of marking a series of symbols on sidewalks and walls to indicate nearby wireless access.
Typically with the SSID written above the symbol and Bandwidth below.
Inspired by the practice of hobos during the Great Depression to use chalk marks to indicate which homes were friendly.

the thing that you don’t have when you’re phones percentage is over 20

The practice of leaving coded chalk marks on or near buildings or sites where a "war driver" has identified available 802.11 wireless network access. The marks usually indicate the type of access and level of security.

This practice is decended from the way hobos used to chalk mark the curbs outside homes where an easy mark could be approached for food or other handouts.

a practice in which hackers draw a symbol in chalk on a wall or pavement to indicate the presence of a wireless networking node.