How frequently did the words 1 and 25 appear together?
How frequently refers to occurrences;  word 1 refers to wid = 1; word 25 refers to wid = 25; appear together means pair, which is a relationship of two words: w1st and w2nd, where w1st is word id of the first word and w2nd is a word id of the second word;  w1st or w2nd = wid = 1; w1st or w2nd = wid = 25;
SELECT occurrences FROM biwords WHERE w1st = 1 AND w2nd = 25