It's a difference of EMAs, as stated earlier by FC. Each MACD value is the difference between the two MA's used, eg 12 and 26 day EMAs... simply subtract one from the other. The signal line is the result of making another EMA up from the series of MACD values for each day, ie an EMA of the differences. MACD going from below zero through zero just means the faster EMA has turned up and gone through the slower - a 'golden cross' (bullish) of the EMAs as many would call that. So the MACD going from below zero to above shows recent price action is trending upward, the slower 26 day ema (assuming 'standard' values) is lagging.
Generally speaking a MACD rising below zero, that is overtaken by its signal line rising through it, will reflect a price chart where recent rises are accelerating - the chart might still look bearish, but the drop is bottoming out. MACD below zero but climbing = short term MA rising above longer, bullish. MACD below zero falling = short term falling faster than long, bear doldrums. MACD above zero climbing = short term accelerating faster than longer, both look bullish, so you are getting into the bull phase a little later than in the 'MACD below zero, climbing' case, and MACD above zero and falling is long term MA is really starting to catch short term up, uptrend is running out of steam.
Dave