What does it mean?
Here is one of the definitions of word 'seal' by Oxford Dictionary:
• the water standing in the trap of a drain to prevent foul air from rising, considered in terms of its depth.

Could somebody please explain what it means?

Question

What does it mean?
Here is one of the definitions of word 'seal' by Oxford Dictionary:
• the water standing in the trap of a drain to prevent foul air from rising, considered in terms of its depth.

Could somebody please explain what it means?

Answer

Hi Vlad,
What they actually mean is a 'water seal trap' ... ==> A trap seal is a pluming term where water is used to both seal a pipe and aid in the movement of water.
Please see 
http://www.wisegeek.com/what-is-a-trap-seal.htm
Kind regards,
  Georg