As the name suggests, Ladder Strings refer to strings that hold the slats in the Venetian Blinds together, as seen in the picture below.
Venetian Blinds with ladder string provides a "floating slats" effect when viewed from far.
This effect proves to be quite trendy when you just want to portray the horizontal slats without any strings attached.
The other alternative to Ladder String will be Ladder Tape. For this option, there will be a Tape that holds the slats together, as seen in the picture below.
The Ladder Tape are usually visible even from far. Even though they are visible, they are aesthetically designed so that they will match the colour of the blind.
When using Ladder Tapes instead of Ladder Strings, it is very common for people to choose use a dark coloured tape (black colour tape) when ordering a light-coloured blind (white slats, or light brown slats etc), and vice versa.