Code is read far more often than it is written, and names carry most of its meaning. A good name says what a value is or what a function does, in the words of the problem, so the reader does not have to decode it.

def calc(l, t):
    r = []
    for x in l:
        if x[1] >= t:
            r.append(x[0])
    return r

def passing_names(results, pass_mark):
    return [name for name, score in results
            if score >= pass_mark]

data = [("Ada", 81), ("Bo", 47)]
print(calc(data, 50))
print(passing_names(data, 50))

Both functions do the same thing. Only the second can be understood without running it.

• Functions do something, so name them after the action or the result: send_invoice, total_price.
• Booleans read as yes or no questions: is_active, has_stock.
• Collections are plural: users, prices_by_item.
• Avoid one-letter names and abbreviations only you know.

Rename f to top_scorers(scores, count) and give every local a real name: it returns the names of the count highest values in the dict scores, highest first, ties in alphabetical order. Rename chk to is_passing(score, pass_mark=50), returning a bool.

The explanation opens here once every check passes.