A support level is a level where the price tends to find support as it falls. This means that the price is more likely to "bounce" off this level rather than break through it. However, once the price has breached this level, by an amount exceeding some noise, it is likely to continue falling until meeting another support level.
A resistance level is the opposite of a support level. It is where the price tends to find resistance as it rises. Again, this means that the price is more likely to "bounce" off this level rather than break through it. However, once the price has breached this level, by an amount exceeding some noise, it is likely to continue rising until meeting another resistance level.
Proactive support and resistance methods are "predictive" in that they often outline areas where price has not actually been. They are based upon current price action that, through analysis, has been shown to be predictive of future price action. Proactive support and resistance methods include Measured Moves, Swing Ratio Projection/Confluence (Static (Square of Nine), Dynamic (Fibonacci)), Calculated Pivots, Volatility Based, Trendlines and Moving averages, VWAP, Market Profile (VAH, VAL and POC).
Reactive support and resistance are the opposite: they are formed directly as a result of price action or volume behaviour. They include Volume Profile, Price Swing lows/highs, Initial Balance, Open Gaps, certain Candle Patterns (e.g. Engulfing, Tweezers) and OHLC.
A price histogram is useful in showing at what price a market has spent more relative time. Psychological levels near round numbers often serve as support and resistance.