Low Profile just means the top of the bed is closer to the floor. Many mattress/box spring sets are so thick now that when placed upon a standard frame a person might feel a step stool is necessary to get into bed. Platform beds which usually only use a mattress set upon a solid wood base are low profile beds. There are also low profile metal frames that are roughly half as high as normal frames. A standard profile box springs is 9" thick.
A low profile box springs is 5" to 5 1/2" thick. Low profile box springs are thinner, but give the same amount of support to the mattress.