What does "know one's stuff" mean?

[Christina328]

English Language Tests, Intermediate level

ESL/EFL Test #54 "Lots of Stuff", question 5

It's quite clear she's studied the subject very thoroughly and when she lectured, you could tell she ......... her stuff.

(a) knew
(b) got
(c) showed
(d) took

English Language Tests, Intermediate level

ESL/EFL Test #54 "Lots of Stuff", answer 5

It's quite clear she's studied the subject very thoroughly and when she lectured, you could tell she knew her stuff.

Correct answer: (a) knew

Your answer was: incorrect
It's quite clear she's studied the subject very thoroughly and when she lectured, you could tell she got her stuff.
_________________________

Dear Sir

what does "know one's stuff" mean?
why not "get" here?

thanks a lot in advance

Christina's son

[Alan]

Hi Christina328,

You asked: