Yes, willfully skipping mass is a grave (mortal) sin. The key word here is willfully - being physically unable to attend, sick, or having other (legitimate) obligations is a valid excuse for not attending. From the Catechism:
The Sunday Eucharist is the foundation and confirmation of all Christian practice. For this reason the faithful are obliged to participate in the Eucharist on days of obligation, unless excused for a serious reason (for example, illness, the care of infants) or dispensed by their own pastor. Those who deliberately fail in this obligation commit a grave sin. (#2181)
The reason is because not attending mass is considered a violation of the third commandment - "keep holy the Sabbath" - and thus a serious offense against God's will.