Poly is having (the option of) romantic and sexual relationships with more than one person at the same time, and the willingness to let your partners have romantic and sexual relationships with other people too.

The first part is the easy one but not everybody has the ability to manage multiple parallell relationships in a good way - lots of people are serial monogamous (mono-romantic) rather than poly and will struggle with handling multiple relationships. If this is you, consider being open to sexual connections only and reserve your emotional connections to one parter.

The second part, letting your partners have romantic or sexual relationships with others, is the harder part because you must be willing to do the emotional work necessary to handle the jealousy and insecurities that this might trigger.