It's a special shooting mode on many Olympus and OM System cameras which buffers, that is configured, with a half press. When you fully pressed the shutter button the buffered images are written to storage + the configured images.
There are 2 pro capture modes, hi and lo. In hi only the focus value is saved where as lo, with C AF, it continues to focus.
I usually use Lo for BiF and other erratic subjects. When the subject wasn't I use Hi. For example when wildlife leaves or arrives to a location.