Chi-square test - how can I say if attributes are correlated?
Chi-Square is used to determine which of the attributes are most informative. Its used in feature Selection.

So, if you have an attribute A ,B and C and output Y, we are trying to know Y depends on A or B or C ? A or B or C might be independent also i.e. has no affect on output Y.

So Chi-Sqaure is a statistical test to find out which attribute is independent and can be removed.

A contingency tables is created for each attribute value and its frequencies/occurrence is recorded and p_values above/below threshold determines if its relevant or not.

More about it here-
https://machinelearningmastery.com/chi-squared-test-for-machine-learning/


  Online, I read about significance level, but i didn't quite understand
  it. In particular, is it at 0.5 or 0.05? Which values are "ok"?


Please read about p-value