The word feature is just a spell check. Grammarly literally checks all the grammar in your writing, recognising tense, sentence structure, words that don't make sense, etc. The free version is just the basic things I mentioned, but the paid version does way more stuff. When you get to University, it would be worth investing in the full version, especially when you are writing scientific essays, projects, papers, etc. Check it out. You'll find the free version helps greatly and notices things you may not even have even noticed.