■ Approximate amount of something. ぐらい or くらい is used for asking "about how much" or "about how many." You can use it for estimating the number of things or time.
HOW to use:
■ Place after the object you are estimating.
■ It is used for estimating objects, time, and also as a pronoun.
About how many customers came?
[Here, くらい is used as a interrogative pronoun: about how many?]
Let me see, about 100 people.
[Here, it estimates the number, "100 people."]
It will begin about eight o'clock.
[Add に to show when it will begin.]
■ くらい and ぐらい are interchangeable and mean the same thing.