Gas is flowing trought the tube from left to right with the velocity v. At the stagnation point (2) the total total pressure is measured. At Point (2) only the static pressure is measured. Thus the difference in levels seen at (3) corresponds to the stagnation pressure.
<a href="//commons.wikimedia.org/w/index.php?title=User:MikeRun&amp;action=edit&amp;redlink=1" class="new" title="User:MikeRun (page does not exist)">MikeRun</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>