In some interlinear translations, the English words which correspond to a series of Greek words are typeset in the same order as the Greek words they represent. However, because English syntax (which is to say, the proper arrangement of words in a English sentence) differs markedly from Greek syntax, the transliteration may not, at first glance, make sense.
For this reason, the translator of the interlinear Bible often numbers the English words corresponding to a Greek phrase, in order to indicate to the reader the sequence in which the English words should appear in a proper English syntax. The translator then uses square brackets to enclose the series of numbered English words, to indicate that the words constitute a single phrase. Such indication is essential when a single Greek sentence contains several phrases which require such numbering of the words of the English translation. Accordingly, several pairs of square brackets may appear in the translation of a single Greek sentence.
In the translation and passage to which you refer, there are four sets of brackets, the first being:
ton tes pisteos archenon kai teleioten iesoun
[2the 6of the 7belief 3head 4and 5perfection 1jesus]