first: the one out in front, whether in a sequence, in time gone by, or in rank
This word names whatever comes first. The single idea behind it is being out in front of everything else, and the context decides in what way. Sometimes the word is counting: the first day, the first month, the first in a line of people or events. Sometimes it is looking backward in time: the former rains, the earlier days, what happened before. And sometimes it is ranking: the foremost person, the leader, the one ahead of all the rest.
These shades overlap, because being first in a count, first in time, and first in importance are really one idea seen from different angles. The same word that numbers the first month can also mean the months now past, and can also crown someone as chief. English needs several different words for these, but Hebrew uses this one.
The word is built from the everyday word for head, and that picture sits underneath all its uses: the head of a line, the head of a stretch of time, the head of a group. When the prophet Isaiah has God call himself the first and the last, he is reaching for both the time sense (before everything) and the rank sense (above everything) at once.