What does this mean?

Her time of 12:32 took four seconds off the mark she set in Friday’s preliminaries.

What the her time mean?

I don't understand off the mark.

And what set in Friday's mean?

AcbeatWhat the her time mean?

What does "her time" mean?

She seems to be an athlete competing in some sort of race. Her time is how long it took her to get from the start to the finish line.

AcbeatI don't understand off the mark.

Don't parse it that way. She removed some time (took it) from (off) the time she recorded before. Her new time was less than her old time, so you can say she subtracted time, she took it off.

AcbeatAnd what set in Friday's mean?

"Set" has the longest entry in most dictionaries. You say a racer sets a time when she runs a race, and her time is duly recorded. And don't parse it "set in Friday's". She set a time in the preliminary races that were run on Friday.

Thank you for your kind answer.

I can understand you.