It's a type of answer that doesn’t really answer your question.
Let me give you an example or two:
If You ask a professional classically trained musician:”-Who are your favorite composers?” And he/she doesn’t really want to answer, he/she might say: ”This question is impossible to answer for me. My stock answer is the composer that I love is the one who wrote the piece on which I am currently working.”
If a semi-/pro poker play is asked by his girlfriend’s parents “what do You do for a living?” And he doesn’t really want say, but he also doesn’t want to lie, he might say something along the lines of: “I’m a short-term high-risk investment analyst.”
Politicicans and big companies use them quite often in their press statements, and public Q&A’s.
Generally answers like “well that’s just how it is” and similar to them are the type of answers You should avoid while analyzing Your thought and emotional processes.
More about analyzing and deconstructing Your automatic thoughts can be found in all sorts of materials about Cognitive Behavioural Therapy and/or negative automatic thoughts (NATs).
ps: At least that's just how I understand "stock answers"