As an opening
In every system, a 3♣ opening is preemptive.
After an opening of one of a suit
After partner opens at the one level, 3♣ is a jump shift. Its meaning varies greatly, depending on the system, but is typically either very strong or very weak.
After a 1NT opening
After partner opens 1NT, 3♣ can have a number of meanings, such as
- Splinter bid
- Puppet Stayman
After partner double's RHO's weak two opening
3♣ is a response bid to Lebensohl.