20" x 2.125" Schrader Inner tube is a large volume thick inner tube.
Trials (19" - ISO387):
The 20" x 2.125 is the inner tube fitted and recommended by the manufacturers of the 19" tyres. If you are not sure of the what a 19" tyre, check here.
BMX/Trials/Freestyle (20" - ISO406):
20 x 2.125" inner tube is ideal for large 20" tyres from 2.125" up to 2.6". This inner tube is ideal for fitting in to the Quax Cross 20 x 2.5" tyre.
If the tyre pressure is low then your wheel will be prone to punctures.
Snake bites are common if the cycle is ridden with the pressure too low.
Blow-outs are more common with worn tyres. It is always useful to have a spare available.
The manufacturer will vary depending on availability.