Before us is Mishnah 4 in the fourth chapter of Maseches Megillah, which deals with the order of the public Torah reading: the number of verses in each aliyah, the manner in which verses are handed over to the translator, and the law of skipping from one passage to another.
"Hakoreh baTorah lo yifchos mishloshah pesukim":
One who is called up to the Torah does not read fewer than three verses, and this is the minimum measure of every aliyah.
"Lo yikra lameturgeman yoser mipasuk echad":
Here the Mishnah deals with the law of the meturgeman, the translator. In earlier times the practice was that a translator stood alongside the reader, and after the reader read a verse the translator would render it into Aramaic and convey it to the congregation. The reader may not hand over to the translator more than one verse at a time: a verse in Hebrew, and immediately its translation. The reason is the concern that if several verses were handed to him at once, he would not translate them properly, and in the Chumash this is especially serious, since the content bears on the laws of the Torah.
In the Torah: only one verse at a time.
In the Prophets: one may read three verses and hand them over together to the translator, since the content is not so critical in terms of halachah and Jewish conduct.
"Hayu shelashtan shalosh parshiyos - koreh echad echad": if the three verses in the Prophets belong to three separate passages, then even though there are only three verses here, each one stands as a paragraph in its own right, and therefore he reads them one by one.
"Medalgin banavi ve'ein medalgin baTorah":
When one is reading a particular section and wishes to skip from it to another section, this is permitted in the Prophets, and one may move from subject to subject. In the Torah, by contrast, one does not skip, out of concern that the reader will become confused, and an error in the Chumash is more serious.
Nevertheless, the Gemara explains that the prohibition against skipping in the Torah was stated only when moving from one subject to another; but within the same subject itself it is permitted to skip from place to place. And indeed this was the practice of the Kohen Gadol, as we learned in the Mishnah, that on Yom Kippur he would continue reading the matters of the day and skip from passage to passage.
"Ve'ad kamah hu medaleg?"
The Mishnah clarifies: even where skipping is permitted - in the Prophets even between two subjects, and in the Chumash within a single subject - what is the measure of distance one may skip? To this the Mishnah answers: "Ad kedei shelo yifsok hameturgeman" - one may skip a span that can be covered in the time it takes the translator to say his translation of the verse. As long as the translator has not finished his translation, there is no burden on the congregation and no lack of respect for the congregation, since they are listening to the translation in the meantime, until the reader arrives at the new place.
In summary: in this Mishnah we learned four laws: an aliyah to the Torah is not less than three verses; in the Torah only one verse is handed to the translator and in the Prophets three, unless they are three separate passages; one may skip in the Prophets but not in the Torah, except for skipping within the same subject; and the permitted measure of skipping is such that the translator will not break off his translation.