The % Net Assets column indicates what percentage of the portfolio's net assets a given security constitutes. Morningstar calculates the percentage of net assets figure by dividing the market value of the security by the fund's total net assets. If a given security makes up a large percentage of the fund's net assets, the fund uses a concentrated portfolio strategy, at least with respect to the security in question. If, however, the percentage figures are low, then the manager is either maintaining per-issue diversification or is simply not willing to bet heavily on a particular security.
Use the % Net Assets column to see how concentrated or diversified a fund is in terms of a particular security. When used in conjunction with the Total Number of Holdings column, this data point can be particularly useful.