Refer to figure.
There are three types of markers installed as part of an Instrument Landing System (ILS), and they can be used for any instrument approach. The marker in question is the Outer Marker (OM), which is at the point on the approach that the aircraft would be around 1500 ft.

Outer marker: (OM)

This marker normally indicates the Final Approach Fix (FAF). It is located between 4 and 7 nm from the runway threshold on the same course as the localiser. When passing the outer marker, the pilot receives an audio tone in a continuous series at 400 Hz, accompanied by a flashing blue light in a series of dashes (2 per second). "- - - - - - "

Middle marker: (MM)

This marker normally indicates the CAT I Missed approach point and it is positioned between 0.5 and 0.8 nm from the runway threshold. When passing the middle marker, the pilot receives an audio tone in a continuous series at 1300 Hz, accompanied by a flashing amber light in a sequence of alternating dots and dashes. "- . - . - . - ."

Inner marker: (IM)

This marker normally indicates the passing of the runway threshold. When passing the inner marker, the pilot receives an audio tone in a continuous series of 3000 Hz, accompanied by a flashing white light in a sequence of dots (6 per second). ". . . . . ."