A constituency link means MPs represent local areas and are directly accountable to those areas for their continued careers.
Voting systems can have stronger or weaker constituency links.
Some Party List Proportional Representation systems have large groups of MPs who are elected as a group to represent large areas of the country. This means that they can’t be held individually accountable.
Likewise, in First Past the Post MPs can be elected with minority support. So whilst they represent a small geographical area, the majority of their constituents might not want them to represent them.
The Single Transferable Vote on the other hand has a strong constituency link as MPs still represent a relatively small area, whilst voters are more likely to have an MPs for whom they have voted for.