Low fidelity is the grey version: outlines, crossed image boxes, lines where body copy goes. It looks unfinished on purpose.
When a screen looks finished, feedback turns into opinions about colour and type. When it looks like a sketch, people talk about what's on the page, what's missing and what order it should come in. That's the conversation you want first.
When to switch to mid
Switch to mid fidelity once the structure holds: real text, filled primary buttons and your accent colour. It's one switch for the whole wireframe, and nothing you typed is lost.
Free accounts can try mid fidelity at any time; frames and exports carry a faint watermark until you upgrade.