IntValues, short for Integer Values, are instances which store integers.
Basically, they're a "thing" which only purpose is to store a number. That number cannot be a fraction or decimal, it must be an integer, or whole number. So you could set it to 1 or 2, but not 1.5.
Since all they do is store a number, you have to get creative. One advantage is that they're an actual instance, so you can pass data between scripts.
They are Roblox instances with a Value property that can hold an integer. You can use them by creating one (via script or insert object) and change the value (via script or properties toolbar).
To create and edit one with a script:
local intValue = Instance.new("IntValue"); intValue.Value = 10; -- Set the value property to 10 intValue.Parent = workspace; -- Put it in Workspace