"About", "roughly", "-ish". Stick it after a number or amount to stop committing to an exact figure — the two spellings mean the same thing.
The short version
Put くらい (or ぐらい) after an amount to make it approximate: 3週間くらい = "about 3 weeks", 5時ぐらい = "around 5 o'clock". It works with amounts, times, prices, distances — anything you'd round.
3週間くらい— about 3 weeks
5時ぐらい— around 5 o'clock
くらい — approximate amounts
The everyday "about". Reach for it whenever you don't want to sound like you're quoting an exact number.
そして今回は3週間くらい。
And this time, about three weeks. (approximate span)
🗣 くらい and ぐらい are interchangeable. There's no difference in meaning — pick either. Rough rule of thumb: ぐらい feels a hair more casual and is common after numbers in speech; くらい shows up more in writing. Don't overthink it.
Don't confuse it with ごろ. 頃 also means "about", but only for points in time — 3時ごろ (around 3). くらい/ぐらい covers durations and amounts too — 3時間くらい (about 3 hours).