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.
In some Party List Proportional Representation systems, you vote for a party and elect a group of MPs to represent a large part of the country, like the whole North West. This means that they can’t be held individually accountable, and may may all live in the same major town.
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.