The icon people are referring to are those that show on your heroes while a buff is active.
Here are some examples:
Or another extreme example:
Often people say “different icon means they stack,” but that’s actually not accurate.
There are some examples of buffs that are similar and therefore don’t stack, but have different icons. Brienne’s Berserker doesn’t stack with a standard attack buff, for example, even though the icons are different.
In contrast, Wu Kong’s Gambler’s Stance does stack with attack buffs.
There’s a list of attributes that stack at the bottom of this infographic: