things that are constitutively and ubiquitously expressed – they’re made all the time (constitutively) at relatively constant levels in “all” cells (ubiquitously). This makes them useful as internal references for qPCR and loading controls for western blots. Basically, you measure levels of a housekeeping gene or protein (e.g. tubulin, actin) in the same sample as the thing you really care about, then divide by the relative amount of the housekeeping protein to control for differences in how much sample you started with.