A well structured piece of writing is made up of paragraphs, each of which usually contains one complete set of ideas.

Linking refers to the ways that paragraphs are linked together, so that the reader can follow your line of thought. For example:

‘Clear vision is an important qualification to get into the army. Army personnel need to have good clear eyesight, or they will fail the medical.

Another area where the army is very strict is in general level of fitness. Recruits must be reasonably fit to get past the medical, and maintaining fitness is a regular part of army life.’

The phrase ‘Another area where the army is very strict’ is a linking phrase, because it refers back to the previous paragraph, and allows the reader to see the relationship between the two paragraphs.