The phrase "bottom rack" refers to the lowest shelf or part of something, often used to describe the lowest level of a dishwasher where dishes are placed for cleaning. Full definition
Place baking stone or inverted baking sheet on bottom rack of oven to heat. (vegetariantimes.com)
Bake at 400 ° for 8 minutes with 1 pan on bottom rack and 1 pan on second rack from top. (health.com)
Keep an eye on them and flip halfway through when one side has browned sufficiently, also rotating the top / bottom racks in the oven. (wander-crush.com)