Volatile functions
Functions that recalculate on every change, whatever their inputs did.
Most functions recalculate only when a cell they depend on changes. A volatile function recalculates on every recalculation of the workbook, because its result can change without any input changing. NOW and TODAY are the obvious cases, and RAND, RANDBETWEEN, OFFSET and INDIRECT behave the same way.
Volatility is contagious: any formula depending on a volatile cell also recalculates every time. In a large model this is the usual cause of sluggish recalculation.
Where performance matters, prefer non-volatile alternatives (INDEX instead of OFFSET, a direct reference instead of INDIRECT), or park the volatile value in one cell and reference it.