Mechanical keyboard is classified by the switch it used, if low profile switch is used, then the keyboard is called low profile keyboard.
What is low profile switch?
Low profile, as it states, is lower in height of the switch, mechancal key switch when it was introduced, came in the height as it called today common switch or high profile switch.
A typical regular (high profile) switch
The low profile switch
In low profile switch, the height of stem and upper case are significantly reduced.
The height of the stem is 3.4 mm in low profile vs 3.6 mm in high profile, which indicates both switches have similiar key travel;
The total height of the part above the plate is 6.4mm in low profile vs 11.0 mm in high profile, which is significantly reduced so that the keyboard can be made compact.
Keycaps are lower too
As for keycaps, there are many keycaps for high profile switches, however whether those keycaps are Cherry, OEM, DSA, etc profiles, their height is higher than 11 mm, so the total height of the top part can't be less than 11mm;
The specially made low profile keycaps are employed in Coyres keyboard, it's total height of 3 mm makes the top part of the keyboard 6.2mm