BTU = 'British Thermal Unit', the common unit for natural gas in Europe and often seen in the US as well.
I'm guessing your utilities are included in your rent. Anyone who owns a home or rents an apartment with its own meter is familiar with BTU from their gas/electricity bill.
http://www.aga.org/KC/ABOUTNATURALGAS/ADDITIONAL/Pages/Howto... - looks like it depends on whether they choose to measure volume or energy content.