Istifal means lowering: the rear of the tongue is not raised toward the palate as it is for an istiʿla letter. This is commonly associated with a lighter sound quality.
Every letter except the seven of istiʿla belongs to istifal. Examples include ت، س، ك، ب. This basic classification does not by itself settle every contextual rule for letters such as ر or ل, which require more specific study.
Contrast: ص belongs to istiʿla while س belongs to istifal. Pronouncing them with an identical heavy resonance erases an important distinction in their identities.