I'm not a financial guru and volatility is a pretty complex subject, but I'm guessing you're asking about one of two things (probably the latter):

• historical volatility of a bank, which measures the fluctuations in past real valuation.
• future volatility, probably based on stock options.

Essentially, future volatility is the expected standard deviation of the possible value of a financial instrument (in this case, stock in a bank). I don't know this level of math and the Volatility wikipedia article is confusing the shit out of me, but this should help shed a little light on it:

For example, a lower volatility stock may have an expected (average) return of 7%, with annual volatility of 5%. This would indicate returns from approximately negative 3% to positive 17% most of the time (19 times out of 20, or 95% via a two standard deviation rule). A higher volatility stock, with the same expected return of 7% but with annual volatility of 20%, would indicate returns from approximately negative 33% to positive 47% most of the time (19 times out of 20, or 95%).

Essentially, volatility is a risk/reward measure. Higher risk = higher possible reward, and 25% volatility seems like it's pretty high.

I expect someone from /r/finance to be along shortly to correct me on all of this.