In a relationship with multiple partners, one of the partners is often in a steady relationship, or both have relationships with other people. The relationship with other partners is often about sex (casual sex), but not always. Polyamory is when someone has an intimate relationship with different persons at the same time. Love, honesty, openness and respect are important in polyamorous relationships.
If you have unprotected sex with various partners, you may run an increased risk of STIs. Use a condom to protect yourself, your partner and other people.
After some time, a relationship with multiple partners can become too difficult, for example because of jealousy.
Some people find it strange to have multiple relationships at the same time, while others find it very normal. That is fine. In Europe it is forbidden by law to be married to more than one person at the same time.