What does it mean when your pelvis begins to hurt during pregnancy?

Many reasons. It depends on how far along you are in your pregnancy. Pelvic pain could be due to a variety of different reasons (infection, preterm contractions, round ligament pain, etc.). Any significant type of pain during pregnancy should warrant an assessment by your physician.
Pregnancy changes. As the uterus enlarges with progressing pregnancy, the person may experience a variety of "aches and pains" in the pelvic region. The majority of these forms of discomfort are normal. Your obstetrician should be able to examine and reassure you.
Getting ready! During pregnancy your placenta releases a hormone called Relaxin that softens your connective cartilage; this will allow your pelvis to open during childbirth but can also lead to pain/hip instability/pubic symphyseal separation, which can be painful. Exercising your stabilizer muscles can help; you may also consider buying a pelvic girdle/belly band for compression/support.