在 ProgressDialog 中，“不确定”是什么意思？

It means the "loading amount" is not measured.

From wiktionary: Indeterminate: Not accurately determined or determinable. It basically just means you're unsure how long the action will take so you cannot say for example something is 50% done.

This normally just means the progress will be displayed as a constantly moving loading bar rather than a percentage or the like.

In simple language we can say when the amount is not determined means we don't know how much it gonna be to be completed or loaded fully..!!

Basically when setProgressStyle(ProgressDialoge.STYLE_SPINNER) is taken then setIndeterminate() will be true because a circle(Spinner) will rotate, which shows that "do not know how much time it is going to take". When take ProgressStyle(ProgressDialoge.STYLE_HORIZANTAL) we will take setIndeterminate() as false since it take a value/percentage bar like how much percentage it was completed with setProgress(value).

Change the indeterminate mode for this ProgressDialog. In indeterminate mode, the progress is ignored and the dialog shows an infinite animation instead.

Note: A ProgressDialog with style STYLE_SPINNER is always indeterminate and will ignore this setting. Blockquote

For illustration, the progress animation keep loading from left to right and then repeat automatically without user interaction(which setProgress) when indeterminate set to true.

Reference here.