Here is a chart from a knitting magazine. Row 83 is the beginning of the armhole shaping. It begins with Bind Off 3.
Then for the next series of Rows, up to Row 91...the armhole is 'shaped', as the chart appears.
This is TYPICAL armhole shaping. It rarely goes in and straight up.
This is a schematic of another sweater I'm making a granddaughter.
Notice that the armhole shaping is also a gradual tapering.
The purpose of the 'shaping' and 'tapering' is to fit the armHOLE around the human's arm. The beginning of the armhole shaping is actually in the armPIT. So the tapering allows it to come around from the armpit and underarm of the human arm, then when it straightens out, that is the part that is visible as a seamline up to the shoulder.