vorderst
frontmost
Describes the position that is right at the front; located furthest forward. It is the superlative form of 'vorder'.
Die Ehrengäste saßen in der vordersten Reihe.
The guests of honor sat in the frontmost row.

foremost
Loading your saved state.
frontmost
Describes the position that is right at the front; located furthest forward. It is the superlative form of 'vorder'.
Die Ehrengäste saßen in der vordersten Reihe.
The guests of honor sat in the frontmost row.

'Vorderst' (frontmost/foremost) is the superlative form derived from the adjective stem 'vorder-' (e.g. 'der vordere Bereich' — the front area). This stem in turn comes from the adverb 'vorn' (at/in the front). It is a fixed, somewhat irregular superlative formation.
Wir saßen im vorderen Teil des Busses, aber nicht auf dem vordersten Platz.
We sat in the front part of the bus, but not in the frontmost seat.
In its literal meaning, 'vorderst' (frontmost) is almost always used directly before a noun and with a definite article, because it describes a unique position. Typical nouns are 'Reihe' (row), 'Platz' (seat/place), or 'Linie' (line).
Die Läufer stellten sich an der vordersten Startlinie auf.
The runners lined up at the frontmost starting line.
In the figurative meaning, 'vorderst' (foremost) is very often used in the fixed expression 'an vorderster Stelle stehen/sein' (to be of foremost importance / to come first). This means that something has the highest priority.
Bei diesem Projekt steht die Sicherheit an vorderster Stelle.
In this project, safety comes first.
You usually recognize the figurative meaning by its combination with abstract nouns such as 'Stelle' (place), 'Front' (front), or 'Priorität' (priority). If it is about concrete, spatial things like 'Sitz' (seat) or 'Wagen' (carriage/car), the literal meaning is meant.
Er kämpft an vorderster Front für den Umweltschutz.
He fights on the front line for environmental protection.
at the back / behind
'hinten' and 'vorderst' describe opposite positions. 'hinten' means in the rear part or at the end of something. 'vorderst' means right at the front, that is, in the first position or the position furthest forward. The grammar is also important: 'hinten' is often used on its own as a location, for example 'Ich sitze hinten' (I am sitting at the back). 'vorderst' usually describes a place or position more precisely, for example 'in der vordersten Reihe' (in the frontmost row).

Bei der Schulaufführung saßen die Eltern in der vordersten Reihe.
At the school performance, the parents sat in the frontmost row.
Im Bus sitzt Max gern hinten, weil es dort ruhiger ist.
Max likes sitting at the back of the bus because it is quieter there.