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.
If you have unprotected sexual contacts with different partners, you may run a higher 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.
In Europe it is forbidden by law to be married to more than 1 person at the same time.