Baby clothing size?

I feel so dumb for even asking this but what exactly does size 3 MONTHS mean? Is it like 0-3 months or 3-6 months? Or something that just says 6 MONTHS is that good for up to 6 months of age or does that last 6-9 month or what here are some examples I picked up.

I’m sorry if this sounds like a dumb question

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