Consider this example.
In a survey of favourite crisps in a class of S1 pupils the following data was collected.

Flavour of crisps --- Frequency
Plain ---------------------- 5
Bacon -------------------- 3
Cheese & Onion ----- 11
Prawn cocktail -------- 6
Tomato sauce --------- 5

A bar chart would be drawn with the flavour of each type of crisp written horizontally across the graph. The vertical axis would be labelled Frequency.

A block graph would look almost identical except that each bar would be divided into blocks.

In this example:- the bar representing Plain crisps would be 5 blocks high.
the bar representing Bacon crisps would be 3 blocks high.
the bar representing Cheese & onion crisps would be 11 blocks high.
etc.

I hope that this helps. Good Luck.