Simply put, distressed investing is an investment style that focuses on investing in companies that seem to be falling apart.
Also, there is a legal aspect to distressed investing (i.e. creditor rights, seniority in the capital structure, collateral, bankruptcy process) that you are not exposed to in traditional equity investing.
For example, say you are looking at investing in the unsecured debt of a company that has $1Bn of first lien debt, $500MM of unsecured debt, generates $200MM of EBITDA and spends $50MM per year on Capex.
If you know a little bit about value investing already, then you should know the basic difference between a good investment and a bad investment (read all the best investing books if you haven’t already).