Boom is a period of time during which the market enjoys a boost in economic growth. It can also indicate extraordinary growth in business activity or sales or a product instead of general economic growth.
For stock market, boom results in bull markets which see significant growth in value of securities. Bust is opposite to boom and it is associated with bear markets. Usually bull markets are followed by bear markets.
Boom simply indicates that stocks are performing well and due to that they gain strong profits. “Dot-com bubble” was a famous stock boom when internet technologies enjoyed rapid rise in stock prices during late 90s.