Welcome to the forum! A flow-check a methodical verification of certain controls/gauges etc related to that aspect of flight. It is call a "flow-check" because it often occurs in a linear fashion, ie starting in at one point, and working methodically through all of your "controls/actions" as a pilot and ending at another point. It is done the same way everytime and often has a "flow".
Checklist are considered mandatory. However, a checklist can be a "to do" list, or a "check" list. As a to do list, you call out the item, do it, and verify complete. A "check" list is using a flow check. Committed to memory, you complete your "flow-check", and then take the checklist and verify that you did in fact, complete all the required actions. GUMPS is a flow-check.
Talk to your CFI about this, it is a very useful tool and will certainly aid you i becoming a proficient and more professional pilot.
Hope that answers your question,