Good question. Now that you have mentioned the instrument name (Agilent 1260 LC), it is easy to locate Agilent's "Refractive Index Detector Agilent InfinityLab LC Series User Manual." It is always good to mention the entire set-up. Some detector display microvoltages on the y-axis on UV detectors.
Intuitively, you can see refractive index can not be negative (you have a negative dead time dip) and refractive index values cannot range in thousands. The signal is definitely not absolute refractive index (n). It is basically $\Delta n$, and in the jargon of detector, it is "Refractive Index Units". Note that refractive index has no units.
The reason for asking for the model was to check what the manual says about the output and what is the scaling factor. It is nano-refractive index units, and the detector is calibrted with respect to a certain concentration of a glucose solution vs. pure water.
On page 19, the manual states,
When sample elutes from the column into the sample cell the refractive
index of the cell contents changes. The change in refractive index
deflects the light beam as it passes through the flow cell resulting
in an unequal amount of light falling on each diode. The change in
current from the diodes that this causes is amplified and used to
produce the calibrated detector signal. This signal expressed, as nano
Refractive Index Units (nRIU), corresponds to the difference between
the refractive index of sample in the sample cell and the mobile phase
in the reference cell.
Secondary note: You have overloaded peak shapes, they should not front. Dilute the sample.