Thanks for clarifying to everyone! :-)

The answers in the comments are good. “Reckon” is very similar to “guess” It’s very casual. I wouldn’t use it until your English is perfectly fluent.

Stan wrote you a great response. I would recommend that you understand the word passively as opposed trying to use it yourself, unless you have as a goal to sound southern or rural. Using this word outside of its normal culture will likely sound like affected speech.

In the US, "reckon" is mostly used in the South, although we all understand it. In colloquial use, it means "to make an educated guess" that is, to make a guess that has a high probability of being correct. Situation: You are driving on the freeway on your way to an important business meeting, but due to an accident, there's a major traffic jam. You call your co-workers and say "I reckon I'll be late" = There's a strong possibility that I'll be late. In more formal English, or written English, it means "to count" or "to tally". It's hardly ever used, but we still find it older books and the Bible. As a side note, "to reckon" is derived from the verb "to recognize".