The tendency for water molecules to stick to one another, as due to hydrogen bonding.
The tendency for water molecules to hydrogen bond to each other makes it difficult to separate water molecules from each other within liquid water. Water as a consequence is said to be cohesive to itself.
See, by contrast, adhesion, which is the tendency of water to stick to other, especially hydrophilic , that is, other than just to itself.
You take advantage of the cohesiveness of water when you water. In the course of siphoning you are creating a column of water that is being pulled from above rather than pushed from below. Just as a must remain intact when pulling up a load, so too that water column that is being pulled up must remain intact, and the word one uses to describe water's tendency to so remain intact while being pulled is cohesion.