In an accumulation fund, the dividends from shares or coupons from bonds held in the portfolio are reinvested within the fund itself, increasing the net asset value of each unit instead of being paid out to the unit holder.
Tax advantage in Spain
As long as you don't sell units, you aren't taxed on that reinvestment: unlike receiving a dividend directly, in an accumulation fund compound capital grows without triggering a taxable event until redemption. This makes accumulation funds generally more tax-efficient for the long term than distribution funds.
Who it makes sense for
Investors who don't need periodic income from the fund and prefer to maximize the effect of compound interest by deferring taxes for as long as possible.