Many people, even native speakers, confuse these two words:
1. it's
2. its
1. The first one is the abbreviated form of 'it is' or - less commonly - 'it has'. The apostrophe (') tells you that there's a letter or two missing.
For example:
It's Monday today. (It is Monday)
It's difficult to learn Japanese (It is difficult)
It's rained a lot this summer ( It has rained)
2. The second one (its) - with no apostrophe - is a possessive pronoun (like my or your) .
The dog is asleep in its basket. (the basket belongs to the dog)
Many native speakers - even some of those who use this site, I'm afraid to say - get this point wrong. If anybody tells you that it should be " it's basket", they are wrong. Check it out in any grammar book.