On Wefunder, a subscription agreement is a contract between you and a WeFund SPV managed by Wefunder Advisors.
Most startups on Wefunder using Regulation D do not allow you to directly invest small amounts in their company. Instead, you are able to invest in a WeFund, which aggregates all the small-dollar investments and invests in the startup as one shareholder. The WeFund holds the underlying security (such as a convertible note, stock, or loan).
When you invest in a Wefund via a subscription agreement, you only have an economic interest - you have no voting or information rights in the startup the fund invests in. You also can't sell any shares in the startup. Wefunder Advisors manage the fund on your behalf and decides when to sell the securities (typically, when the startup is acquired or goes IPO). Only then do you earn a return. This is a very long-term investment. For instance, if you had invested in Facebook in 2004 with a WeFund, you would have had to wait 8 years later until they went public in 2012 to receive a return.
Read the WeFund SPV FAQ.