A graph answers one simple question over and over: how much, for each thing? The "things" run along the bottom (the days, the months, the brands) and the "how much" goes up the side. Read one bar and you've read one fact.
Reading a Single Value
Pick a bar in the toy — say Wednesday. Run your eye from the top of that bar straight across to the scale on the left. Where it lands is the value: that's how many ice-creams sold on Wednesday, full stop. A line graph works the same way; you read the height of the dot instead of the top of a bar. The amber guide-lines in the toy do exactly that trace for you, so you can see the reading happen.
Why the Scale Is Everything
The bars only mean something because of the numbers up the side. Take that scale away and a tall bar is just a tall rectangle. So the very first thing you do with any graph isn't look at the bars — it's read the scale: what's being counted, and how big is each step? Get that, and every bar tells you its number.
Bars Versus Lines
Use a column graph when the things along the bottom are separate categories — Monday, Tuesday, Brand A, Brand B. Use a line graph when they flow into each other over time, because the line shows the journey between readings, not just the readings. Same skill to read either: height tells you the value.