SRID

You can view the SRID information in existing SDO_GEOMETRY columns with the following

SELECT g.GEOMETRY.SDO_SRID FROM MY_TABLE g;

assuming that MY_TABLE has a sdo_geometry column called geometry.

Well, what is the srid?

SRID stands for Spatial Reference Identifier.

Without setting a SRID all values in Oracle spatial are treated as Cartesian.
Using the UNIT parameter will never be supported (for SDO_NN, SDO_WITHIN_DISTANCE, BUFFER, LENGTH, DISTANCE, AREA, etc).
Coordinate system transformations will never be supported, so you won't be able to use a query window or compare to a geometry that has an SRID set.
Most/all tools support spatial data with a NULL SRID.

The SRID allows the correct projection, transformation and relational operations between different co-ordinate systems. The most commonly used SRID in Oracle Spatial is 8307 which is the whole earth projection WGS84 using latitude and longitude.

The complete list of Oracle SRIDs can be found in the MDSYS.CS_SRS table.