Headhunting in terms of employment is when an employee is contacted by another company about the possibility of work for them. This is often done on the quiet and may just be a comment made to an employee or actually inviting them in for an interview or chat. This is often seen as quite an under handed technique by many employers but happens extremely often.
If you are approached about applying for a position within another company then you will need to consider if you may move jobs? If not, then make sure that you reply to say that you are not currently looking to move as you are happy in your current position but you will contact them should you decide to move on. It may always be worth going to have a chat with them just to see what the potion is and the conditions such as working hours and pay etc., you never know could be the best move you have ever made.