Toki pona is a constructed language created by Sonja Elen Kisa, a linguist and translator, at 2001.
The language only has officially 125 words and 14 different letters, being the shortest language that exists, but sometimes communities of Toki Pona learners have tried to introduce new words to it, to be able to express their thoughts in a easier. Even at the first version of the language there were only 118 root words!
The purpose of the language was to be brain exercise or yoga for the brain rather than to be used as an international auxiliary language. I can totally relate to this idea because I tried to learn it once and it gets too much brain effort to speak in that language. Trying to speak relatively fluently was a challenge because with 125 words it very difficult to express any complex idea, you need to create word combinations and variable meanings to one of the 125 words and get that word to fit into a new concept from scratch, at the moment. So... you have to think really quickly and need a lot of practise to make a sentence with the correct grammar and the correct meaning.
Also, for the listener or reader it's very difficult to understand quickly and exactly what the interlocutor meant. One sentence could have 4 or different meanings, depending on the interpretation of the sentence.
If you want to learn the language, you can read the original book published in 2014:
Toki Pona: The Language of Good
Also, you can check the dictionary from the official website: http://tokipona.net/tp/classicwordlist.aspx
If you surf the internet you will see there are a lot of groups of learners in social media as well.