Investing in a mutual fund is like an investment made by a collective. An individual as a single investor is likely to have lesser amount of money at disposal than say, a group of friends put together. Now, let's assume that this group of individuals is a novice in investing and so the group turns over the pooled funds to an expert to make their money work for them. This is what a professional Asset Management Company does for mutual funds. The AMC invests the investors' money on their behalf into various assets towards a common investment objective.
Hence, technically speaking, a mutual fund is an investment vehicle which pools investors' money and invests the same for and on behalf of investors into stocks, bonds, money market instruments and other assets. The money is received by the AMC with a promise that it will be invested in a particular manner by professional managers (commonly known as fund managers). The fund managers are expected to honour this promise. The SEBI and the Board of Trustees ensure that this actually happens.