I reckon is exactly the same as I think, but it is more commonly used in informal situations. It's used all the time in England.

"I reckon" is an expression used only in the rural parts of the US. It would sound strange and unnatural in most situations. As a learner, I would suggest you don't use it and just know the meaning were you to read or hear it. Stick with "think" or "suppose" -- which also have slightly different meanings. Let me know if you'd like an explanation of the subtle difference there.

I reckon is slang for I think I will do that. Even in our part of Texas which is rural, it is not used very much. As Ed said, I would not use it as if you were in a cosmopolitan (City area) you would be given strange looks. Its something that is said between friends.