“Teaching them to observe all things whatsoever I have commanded you:
and, lo, I am with you alway, even unto the end of the world. Amen.”
King James Version (KJV)
A helper word the translators felt necessary to give the full sense that the translation should impart to the English reader that the Greek phrase indicates. The translators cast the word in italics so the reader could know that it was deliberately added, and if one feels it unnecessary, just strike it out. The word is not embroidery.
On the other hand, the translator using the word "observe" for the Greek tereoh could be replaced with the phrase "keep watchfully secure" thus removing some of the ambiguity of "observe" in which case nothing should or would be italicized, and nothing of the implicit sense changed.
But this is not the point. What is not to be added to, diminished, changed, or embroidered is the Greek text itself, which has been done thousands of times by the supposedly "scholarly" critics in their manufacture of a synthetic text presented in 1891, but never before seen by human eyes other than theirs.