Bestinvest says
The fund provides a simple, low cost and flexible way of investing in the UK stockmarket by tracking the FTSE All-Share Index. Whilst the index is broad in nature it is still dominated by large companies in terms of market value (some 80% of the index) and it can be concentrated by sector. Active fund managers have historically struggled to add value at the large cap end of the UK equity market, making low cost passive funds like this an attractive alternative as part of a diversified portfolio.
The method of replicating the index is called partial representation or sampling. This means that the fund does not by every stock in the index, and will take a "basket" approach to some stockmarket industries in its attempt to replicate it. The FTSE All-Share Index is a market capitalisation weighted index and therefore some stocks have a greater weighting and therefore greater effect on performance. By only engaging in partial replication it is possible to keep the costs down however, the tracking error (divergence from the index as measured by standard deviation) will be higher than a fully replicating index.