Clean Tweets: What are UTF8 and non-UTF8 characters

I am attempting to analyze a corpus of tweets extracted from Twitter. A number of tweets appear in non-UTF characters.

For example, one tweet is: "［米国］一人ã®ãƒ¯ã‚¯ãƒ ン未接種ã®å­\ ã©ã‚‚ã‹ã‚‰åºƒãŒã£ãŸéº»ç–¹ã€ã®æ•™è¨“。 @ShotbyShotorg: How one unvaccinated child sparked Minnesota measles outbreak \"

I am not familiar with these non-alphanumeric characters or how to convert/exclude these characters. Are these garbage characters or do they need to be converted? Thank you.