The phrase "timer rings" means that a timer or alarm is making a sound to signal the end of a set time period.
Sentences with «timer rings»
Stir it occasionally and let it simmer on low until the the cake timer rings. (landsandflavors.com)
When timer rings, stir yeast mixture well. (remonline.com)
Scoop the batter into the prepared wells and let the muffins sit until the timer rings so the whole grains can absorb the liquid in the batter. (cookingwithauntjuju.com)