When a guy says "I'll let you know" what does it mean?

Anonymous
He is slightly interested in me.. I texted him and asked him if he was free this week. And he replied:

That's a good question, I'm really busy this week I'll let you know.

Im not completely into him because I dont know him.. We hung out once before and he said he wouldn't mind hanging out once a week or every other week..

Some people actually mean they will let you know. I've said it so many times and I actually meant it. So has my friends and other people.. But others say it's to say they just don't want to hang. Either way idc because I'm not tied to him in any way but I'm curious.
When a guy says "I'll let you know" what does it mean?
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