Answer: It usually means to be selected by God to do some form of full-time ministry. The bigger question is how that happens? Undoubtedly God has put it on the hearts of some individuals that they are to pursue full time pastoring or missions, etc. But this should always and will always be accompanied by gifts God gives to these individuals that are recognized by faithful members of the Body of Christ. An example of this is Acts 13:1-3 where Paul and Barnabas are said to be called by God and through prophetic utterance the church is told to set them apart for missions ministry. But they have already been demonstrating gifting and faithful service for years. Can someone resist this calling or forfeit it by misconduct? I think so. Paul said he labored to maintain faithful ministry lest he be disqualified (1 Corinthians 9:27).
In one sense every Christian is called. We are all subject to obey the great commission Jesus gave us before he ascended to heaven (Matthew 28:19,20). We may also say we have a career calling. But the call to carry out the great commission is the highest priority. Nevertheless, God often wants us to use our career calling to help establish our witness and enhance our ability to carry out the great commission. Some are called to make doing the great commission their entire focus, usually with the idea of helping the rest of the Body become more equipped to do their part in making disciples of all nations (Ephesians 4:11,12).