An investment thesis is basically a hypothesis about what sort of companies will do well as investments. Some investors don't have a specific thesis, and will invest in any company that looks good to them. Others have a specific take on the startup ecosystem. For example an investor might believe that infrastructure companies are a great bet going forward, or they might think that the sharing economy is the right bet, or, as in case of my fund, they might think that companies building valuable and proprietary datasets (and tools for working with those datasets) present the best opportunities. If an investor has a thesis, they will invest exclusively or mostly in companies that fall within that thesis.