That indicates that the format you are using is not supported by hardware accelerated realtime playback and AE generates a RAM preview, whose length as the name implies depends on available memory. If it's unintentional, start by checking your preview options, hardware acceleration settings and graphics driver. Anything more will require specific information about actual settings, contents of the project, your computer and so on, none of which you have provided so far.
I got that at some point. The project now uses only shapes and text (created in AE). My computer should allow more previews (gtx1060 6gb, I7-7700k and 16gb ram). Can you show me which switch will save my life? :C Also, if you put the preview not in the best quality, and there is half, then the preview gets a little bigger