Growth Mindset is the name for a hypothetical mental state wherein an agent believes that they can grow as a agent, and this belief causes the agent to try to grow, which might plausibly lead to more growth!
This is a very natural and obvious hypothesis that could apply to agents in general, and could be defended as a sort of useful philosophic presupposition to often start with, in most domains, about most agents, and most positive traits, that would lead to improvement behaviors that might be quite cheap, and might be quite valuable to have tried!
The primary academic popularizer of the idea within academic psychology is Carol Dweck, who has been embroiled in some controversies.
Sometimes the idea is taken farther than is empirically justified when psychometric rigor is brought to bear on specific traits in specific situations where people might honesty (or dishonestly!) disagree about whether growth or change is possible.
In these debates, sometimes people with strong specific arguments about specific limits to specific growth in specific people are treated as "just not having growth mindset" and then their specific arguments are seemingly speciously dismissed.
Like most concepts, the concept of growth mindset can be abused by idiots.