Exposure compensation is the correction the photographer applied to the meter's suggestion, in EV. +1 means one stop brighter than the camera wanted, -2/3 means two thirds of a stop darker, and 0 means the meter was left alone. EXIF stores it in tag 0x9204 of the Exif IFD as a signed rational.
The tag goes by several names. The EXIF specification calls it ExposureBiasValue, ExifTool calls it ExposureCompensation, and Windows Explorer labels it "Exposure bias" in the file's Details tab. Only the ExifTool name works on the command line: -ExposureBiasValue=1 gets you "Warning: Tag 'ExposureBiasValue' is not defined" and "Nothing to do."
How ExifTool prints exposure compensation
ExifTool prints exact thirds and halves as fractions and anything else as a decimal, so the same nominal value can look different depending on how the camera or editor stored it:
$ exiftool -overwrite_original -ExposureCompensation=-2/3 photo.jpg
1 image files updated
$ exiftool -G1 -s -ExposureCompensation photo.jpg
[ExifIFD] ExposureCompensation : -2/3
$ exiftool -G1 -s -n -ExposureCompensation photo.jpg
[ExifIFD] ExposureCompensation : -0.6666666667
$ exiftool -overwrite_original -ExposureCompensation=0.67 photo.jpg
1 image files updated
$ exiftool -G1 -s -ExposureCompensation photo.jpg
[ExifIFD] ExposureCompensation : +0.67If you filter a library by this field, use -n and compare with a tolerance. Matching the string "+2/3" misses every file stored as 0.67.
A non-zero value tells you the photographer overrode the metering, nothing more. It says nothing about whether the result was well exposed. Read it together with ExposureProgram: in aperture or shutter priority it changed the exposure, while in full manual with a fixed ISO the photographer set the exposure directly anyway. Flash compensation is a separate setting that standard EXIF doesn't record; Canon keeps it as FlashExposureComp in the maker notes.
To change the value, the EXIF editor takes it as free text in JPEG and TIFF files: -2/3, -0.7 and 1 all work. Other ways to change it are in how to edit photo metadata. The reader also opens WebP files, which carry EXIF in their own EXIF chunk when the exporting app keeps it.