All polarisers set to their maximum affect cause a loss of at least 2.0 stops (actual EV stops). Onboard TTL meters will observe this and make the necessary compensation (not necessarily always the right amount of compensation), but with a manual meter (hand-held metering) it's up to you to factor in the compensation value and keep that in mind when the shot has full polarisation applied.

Be aware that a polariser must be matched to the type of camera meter to avoid derangement of the reading; most modern-day evaluative/matrix/multipattern meters require circular polarisers; plain TTL meters can use either linear or circular. Additional correction based on your experience is often necessary above what the camera automatically compensates because very often a polariser can 'flatten' the scene making it look very dead. A little over-exposure is better than under exposure.