At higher grades you must be able to discuss with the examiner musical features such as texture, form, style, and period of a piece of music. My own web pages to help with these parts of the test at Grade 5+ and at GCSE are available here!
At Grade 5, it is sufficient to be able to identify and use the main chords used in cadences.
These are chords:
I tonic 
the first and last chords in this sequence are tonic chords
IV subdominant 
the second chord in this sequence is the dubdominant
V dominant 
the third chord in this sequence is the dominant
In your own compositions you will probably have been adding chord ii to the mix, and you will have been encouraged to spot chord vi in an interrupted cadence.
With this palette it is possible to harmonise most tunes in a simple and effective way. At Grade 6 however you will be called upon to recognise and use triads built on all the notes of the scale. You will also add a 7th above the root to some of your supertonic and dominant chords.
You will have heard the 'magnetic pull' between the dominant and tonic chords in music. Adding the 7th strengthens this effect because of the need for the 7th to resolve to the third in the tonic chord.
You can also strengthen the pull between the supertonic and the dominant by adding the 7th. When this chord is in first inversion it produces a satisfying bass line too.
