Close position means all the notes are packed within one octave, stacked in ascending order: root, 3rd, 5th, 7th. It sounds compact and clear — every note is close to its neighbours. Close voicings are the foundation that drop 2, drop 3, and 4-way close voicings are derived from.
Root (1P) + 3rd + 5th + 7th, all within one octave. For extended chords, the 5th may be replaced by a 9th, 11th, or 13th.
As a reference position from which to derive other voicings. In two-handed chord playing where both hands cover the keyboard. Close voicings sound clean but can feel thick — they work best in mid to upper register (octave 4-5).