In ForEx markets a breakout occures when the price of a currency pair moves above a resistance level or moves below a support level.
A commonly used definition says that a movement characterizing a breakout should be a sudden and directional one, usually followed by an increase in volatility and heavy volume.
No price is too high to buy and no price is too low to sell, identifies the right strategy to trade a breakout. The trouble in identifying breakouts is subjectivity, as the identification of support and resistance levels is completely subjective, as it varies from trader to trader.
More specifically, an upside (downside) breakout is one where the price starts tranding higher (lower). This is a hint for a trader to go long (short) or exit short (long) positions.