Body capacitance is a physical property of the human body that makes it act as a capacitor (see the article; humans conduct electricity somewhat, and do have capacitance as well). Like any other electrically-conductive object, we are able to store electric charge if insulated. We all have a certain amount of capacitance. It is the capacitance between a human and its surroundings that is called body capacitance. The actual amount of capacitance varies with our surroundings; it would be low when standing on top of a pole with nothing nearby, but high when leaning against an insulated, but grounded large metal surface, such as a household refrigerator, or a metal wall in a factory.
Much of the time, we are unaware of capacitive effects. Synthetic fabrics, often very good electrical insulators, and friction as we brush against other objects, in particular, can charge us with respect to ground (earth). Unless we are charged to roughly 3 kV or so, we are essentially not aware of being charged. However, poorly-designed electronic devices (or almost any of them, with the housing removed) can be damaged or destroyed by modest charges, lower than 100 volts. Partly for that reason, electronics factories are very careful, in particular, to prevent people from becoming charged up. A whole branch of the electronics industry deals with preventing static charge build-up, in good part from routine human activity, and protecting products against static discharge when it occurs elsewhere.
Notably, a combination of footwear with some sole materials, low humidity, and a dry carpet (synthetic fiber in particular) can cause footsteps to charge a person's body capacitance to as much as a few tens of kilovolts with respect to the earth. The human and surroundings then constitute a highly-charged capacitor. A close approach to any conductive object connected to earth (ground) can create a shock, even a visible spark.
Body capacitance was a significant nuisance when tuning the earliest tube ("valve", in the U.K. and elsewhere) radios; later designs made them insensitive. However, body capacitance is very useful in Theremins, musical instruments in which it causes slight frequency shifts of that instrument's internal oscillators. One of them changes pitch, and the other causes loudness (volume) to change smoothly between silence and full amount.
Delta makes some faucets that sense the tiny capacitance between a hand that's close to the faucet and the faucet, itself, to operate the faucet.
Some elevator floor-selector buttons sense a finger's capacitance.
Capacitance of a human body in normal surroundings is typically in the tens to low hundreds of picofarads. That's small, by typical electronic standards. While humans are much larger than typical electronic components, they are also mostly separated by significant distance from other conductive objects. Although the occasional static shock can be startling and even unpleasant, the amount of stored energy is relatively low, and won't harm a healthy person. The Human Body Model for capacitance, as defined by the Electrostatic Discharge Association (ESDA) is a 100pF capacitor in series with a 1.5kΩ resistor[1]
A capacitive touch sensor responds to close approach (but not force of touch) of a human body, usually a fingertip. It's the capacitance between the device itself and the fingertip that makes it respond; naturally, electronics senses the capacitance. Capacitive touch screens don't require applying any force to their surfaces, which makes them easier to use in some respects. Futhermore, because of body capacitance, people act as good antennas, and some small televisions use people to enhance reception. [1]