A bubble chart is a scatter plot (dots placed by two numbers) where each dot's size adds a third variable.
It lets you show three numbers per point at the same time. (e.g. (x sales, y profit, size = staff))
Bigger bubbles mean a larger value of the size variable. (e.g. population 1M vs 10M)
Good for comparing items on two axes while also noting their scale or size.
Too many overlapping bubbles can get crowded and hard to read.
In plain words
Like a scatter plot — one dot per item — but bigger dots mean bigger values of a third number.
Where you'd use it
Plot countries with income left-to-right, lifespan up-down, and bubble size for population. A bubble chart shows three facts at once — you might see a huge bubble (a populous country) sitting at low income but middling lifespan.
For products, place price against star rating and size each bubble by units sold; a big bubble in the cheap-and-highly-rated corner marks your best seller.
Charting cities by temperature versus rainfall with bubble size as population lets you compare climate and city scale together in one picture.