I am 28 weeks pregnant & my ob said my cervix feels shorter, what does this mean?

Probably normal: As your pregnancy progresses, your cervix will by necessity shorten. Keep a lookout for contractions, but this sounds normal.

Needs monitoring: This could be normal but also a sign of risk for preterm labor or early delivery. You probably will be seen more frequently to monitor this. You should be aware of other signs like cramps, contractions, increased pelvic pressure or increased discharge.