Asset Management Companies are firms that invest pooled funds from clients, putting the capital to work through different investments including stocks, bonds, real estate etc.
Along with high-net-worth individuals, hedge funds, pension plans, they also manage small investors for whose benefit they create pooled structures such as mutual funds, index funds, or exchange-traded funds (ETFs), which they can manage in a single centralised portfolio.
Since they have a larger pool of resources than the individual investor could access on their own, AMCs provide investors with more diversification and investing options.

Example:
AMCs are popularly referred to as money managers or money management firms. The AMCs that offer public mutual funds or ETFs are also known as investment companies or mutual fund companies for example SBI Mutual Fund, ICICI Securities, Aditya Birla Sun Life Mutual Fund etc.