A step down fix is due to some sort of obstacle in the approach segment or some sort of political thing eg flying over built up areas etc.
It is usually defined as an altitude with a distance from an approach aid or another aid.
It means while you decend on your approach your not allowed to decend below that altitude until after that distance/aid has been reached. After you have met the restriction and past the distance defined or aid on the plate you can then decend to the next restriction which could then be the MDA or missed approach pt or another step down. But there is only meant to be one between FAF and MDA/missed apt point.
There is no need to see the offending obstacle you just have to make sure you don't decend until after the restriction. Its a not below restriction so it is possible to go over the top of it with several hundred feet to spare.
http://edj.net/cgi-bin/echoplate.pl?...MCO_vdr18R.gif
use this plate if you have any more questions then everyone can see what you are on about. As you can see from that plate you have three step down restrictions from the platform height but only one after the final approach fix which is the wee cross.
It doesn't help that there are 3 different flavours of plates in the uk and they all display the information differently