Objective means "based on facts, generally recognized as true": "un jean neuf" was never worn, it just gets out of the factory, it is brand new.

Subjective means "based on opinions/perceptions, relative": "un nouveau jean" has been worn at least once, it was recently bought, when you bought it, it was new or second-hand.

Not all adjectives can be used subjectively (before the noun) or objectively (after the noun), but some have a subjective meaning anyway (BANGS) and others an objective meaning anyway (colors).

• Un petit morceau de gâteau (subjective): what seems "small" too some may be "big" for others.
• J'ai un pantalon jaune (objective): "yellow" is "yellow" even if there are various shades of yellow.
• Dans un récent article (subjective), il a dit...: I don't have the date but it was published what seems to me "not long ago".
• Dans un article récent (objective), il a dit...: I'll give you the date it was published if you ask me.

Some adjectives can change meaning when they are placed before or after a noun:

• Le pauvre homme (subjective) a perdu son portefeuille: I pity him because he has lost his wallet.
• Les hommes pauvres (objective) ne peuvent se faire soigner: They cannot afford to seek care.