This is the 3034th most frequent Polish word.
piąta
"Fifth" or "five o'clock" (depending on context).
In this context, 'piątej' refers to the time 5:00, used in its locative form to specify the time of meeting.
Spotkajmy się o piątej.
Let's meet at five o'clock.
Here, 'piąta' is an ordinal number, meaning 'fifth', used in the nominative case feminine gender to describe the attempt.
To jest moja piąta próba.
This is my fifth attempt.
In this sentence, 'piąta' is used as an ordinal number modifying 'osoba' (person) to signify the position in the sequence.