Manual investing means that the investor him or herself picks loans or investment items, such as houses, he or she wants to invest into. Normally the market places provide you a list of loans with all kinds of parameters: loan originator, interest rate, country, and so on.
The loans that you can see here represent the so-called primary market. The primary-market is the market where all loans or securities are tradet for the first time. You could also say that they are created on that market.
For sure, manual-invest is the most time consuming form of investment. When dealing with consumer loans, where high degree of diversification into various borrowers is key, manual-invest may not be the first choice for first-time investors.