Shorthand for optical-to-electrical-to-optical, a conversion that takes place in some optical transport gear.
ROADMs don't do this; they keep traffic in the optical domain, as do all-optical switches.
The perceived disadvantage to all-optical transport is that there's no opportunity to peek into the wavelength to see the traffic itself. This means, for instance, that the traffic can't be groomed, not until the wavelength is terminated and the data converted back to electrical form.
That's why some optical gear converts the traffic to electronic form, makes decisions or modifications as necessary, then puts it back onto a wavelength. O-E-O.