Hey there,
They effectively refer to 'stage of investing' - so where are you at in your investment cycle. Early stage investor means that you are just starting out, likely younger in age and have a long-term runway. Because of this, you are likely to have a higher risk tolerance or can afford to take on higher risk.
Late-stage refers to those that are closer or at retirement, likely older may be less risk averse and have a greater need for capital preservation.
Mid-stage is somewhere in between.
We used to have age designations (millennial, gen-x, boomer) but it lead to more confusion because everyone's situation is different. Someone who is 40 might be a late stage investor because they are focused on capital preservation, maybe even close to retirement and has a lower risk appetite. Another 40 year old, might be right in the middle and finally, one might just be starting out - not planning on retiring for another 20+ years and is looking to be more aggressive.
Hope this helps.
Mat