A designer collection is a group of pieces built around one idea, using one fabric story, drafted from one set of blocks. That is the technical answer. The practical answer is that everything in a collection is designed to work with everything else in it, so a jacket from Look 04 will sit correctly over a dress from Look 01 without you having to think about it.
What it does not mean here is mass production. I cut each style in a short run, usually ten to fourteen pieces, and when a cloth is finished it is finished. Some pieces exist only once because I bought a single length of something I could not leave behind.
It also does not mean the garment arrives finished and final. Every collection piece is fitted to you before you take it home, and the standard adjustments are included in the price. If a piece needs to be rebuilt around your measurements rather than adjusted, that becomes a custom tailoring commission and I will tell you honestly which of the two you need.
The short version. Ready to wear that has been drafted properly, made in small numbers, and fitted to you before it leaves.