The continual buying and selling of different securities or investments to maintain the proper allocation of one's portfolio. This is contingent upon the desire make up. For example, some might allocate securities based upon 60/40 stocks to bonds. Of the ratio gets too far out of alignment, to the point stocks make up 70%, then selling of those will commence and be replaced with bonds.
Institutional funds often has this as part of the make up of the fund. If it gets overweight in one stock, it will sell some down. This often occurs after the stock has experienced a major run up in price, affecting its ratio as compared to others in the portfolio.