MQ5 Gas Senor module - How to Callibrate
The ppm is parts per million so it is recommending calibrating using a gas that contains 1000 parts per million of the gas the sensor is detecting. There are products called "calibration gas" made for this specific purpose. For example a Google search for "hydrogen 1000 ppm calibration gas" yielded the following result:

Calibration Gas Cylinder, 1000ppm Hydrogen (H2) / Air, (6D) 103 ltr, CZF7E290152

You might be able to find some cheaper / smaller altenatives but most I've seen tend to be sold in reasonably large cylinders and aren't all that cheap for hobbyist purposes.

While the datasheet isn't that clear figure 3 shows that it seems to have a fairly linear response depending on the ppm concentration. According to the Atmosphere of Earth aricle on Wikipedia the concentration of hydrogen in the atmosphere is 0.55 ppm by volume. So for practical purposes you should be able to treat exposure to air as close to 0 ppm and calculate and extrapolate the slope between that and 1000ppm to calculate other values.