Broker/analyst recommendation that could mean slightly different things depending on the broker/analyst. In general, it means to increase the number of shares of a particular security over the near term, but not to liquidate other parts of the portfolio to buy a security that might skyrocket. A buy recommendation, but not an urgent buy.
It refers to buying often coincident with market bottoms or consolidations. It can also refer to purchases by insiders or major investors on a large scale over a period of time. Such accumulation may also indicate the first phase of a bull market. While most investors are discouraged with the market, and earnings are at their worst, some investors start buying shares.