The simplest answer is that a novella-in-flash is a short novel that’s composed of linked but individual short-short stories – each section / chapter / story is up to 1,000 words each. Classic examples of the form include Gwendolyn Brooks’s Maud Martha (1953) Sandra Cisneros’s The House on Mango Street (1984), and Justin Torres’s We The Animals (2011). Longer works in this form are called novels-in-flash or sometimes “flash novels”.
For more information have a read of the following:
Interviews with Winners and Runners-Up of the Bath Novella-in-Flash Award: