It says on Kuwait Airways website that 3 dimensions shall not exceed 158 cm. Does this mean the length, width and depth can be any size, just that they need to all add up to 158cm?
I have never been abroad so I'm new to this.
It simply means:
Length + Height + width
The total of that should not exceed the allowed size. In your case it is 158 cm. BTW that's called linear length.
As noted on their website:
the total sum of the 3 dimensions (Width + length + height) shall not exceed 158 cm (62") for each bag piece.
Additionally, for economy class passengers, the total linear dimensions of two pieces of checked baggage shall not exceed 273 cm (107"), so if your first bag measures 158 linear cm, the second one cannot exceed 115 cm.
There are no limits on any single dimension, so if you had an unusually tall bag it should be fine provided it is shorter or shallower to make up for it.
Note that dimension limits do apply for cabin luggage:
The maximum size is usually 23 x 36 x 56 cm (115 cm total) or 9 x 14 x 22" (45" total)