An income fund is a fund that invests using an income investment style. Typically, it will be invested in a diversified portfolio of stocks with relatively high dividend payments or generate returns through interest-paying assets like bonds.
The alternative is a growth fund. The difference is that the income fund focuses on deriving earnings for investors through the dividends of the stocks or interest payments from the other assets in which the fund is invested while the growth fund tries to grow the original sum mainly through share price or capital appreciation.