Concentration of measure

Concentration of measure is like if you had a jar filled with colorful balls. If you wanted to find one particular ball, like a red one, you would need to look through the jar very carefully until you find it. Concentration of measure is a way of saying that if you have a big jar filled with a lot of things (like balls), then it's more likely that each of those things (like each of the balls) will be similar to each other, rather than really different.