John Gill Commentary Exodus 38:26

"a beka a head, [that is], half a shekel, after the shekel of the sanctuary, for every one that passed over to them that were numbered, from twenty years old and upward, for six hundred thousand and three thousand and five hundred and fifty men." — Exodus 38:26 (ASV)

A bekah for every man, [that is], half a shekel, after the
shekel of the sanctuary
A half shekel was called a "bekah", from (eqb) , "bakah", to divide; because it was a shekel divided into two parts.

for everyone that went to be numbered, from twenty years and
upwards ;
in order to give a ransom, and make an atonement for their souls, as was ordered (Exodus 30:12–15).

for six hundred thousand and three thousand and five hundred and
fifty [men] ;
so that from the time of their coming out of Egypt, which was now about six months ago, there was an increase of 3550 of the above age; see (Exodus 12:37).