Popping Power is a term that describes effectiveness of a Tower at harming bloons, usually by directly damaging them. It is often measured as a combination of pierce, damage, attack speed, among other factors. An attack with more popping power has "stronger" in-game stats. Generally speaking, towers with better popping power have an improved combination of pierce, damage, and attack speed.
Attacks are often associated with those that directly damage bloons, but some attacks purely apply status effects, other indirectly offensive effects, or inoffensive effects. For offensive attacks, popping power is approximated as DPS (short for "damage per second"). DPS is a simpler term that describes only a combination of pierce, damage, and attack speed of an attack, including damage-over-time if applicable.
In the Bloons TD series games, "popping power" is an ambiguous term for pierce, attack speed, or general optimization of a tower. Certain in-game upgrades refer to "popping power" in such a way that doesn't distinctly describe a specific game mechanic (e.g. Full Auto Rifle doesn't increase pierce or damage but its upgrade description refers its statistical improvements as "incredible popping power").
All factors that contribute to "popping power" of an attack include: