The surface tension of a liquid such as water is "the elastic tendency of a fluid surface which makes it acquire the least surface area possible" (Wikipedia). It is the result of the attraction of water molecules to each other (cohesive forces). You can see this in action when you fill a glass of water to the brim: when you pour the water in carefully, you will be able to fill the glass so that the surface of the water is just a bit higher than the brim.
The same cohesive forces also make sure that drop of water or, in Mur Lafferty's story, blood, can float through the air in zero gravity without bursting at the slightest touch. See for example this image of astronaut Clayton Anderson watching as a water bubble floats in the middeck of space shuttle Discovery.
When the droplet does not burst, the surface tension "holds", i.e. the drop's surface remains intact. This is why the orb of blood bounces away instead of blending into the synth-amneo fluid.
(Synth-amneo fluid is probably synthetic amniotic fluid used to preserve cloned bodies until they are needed.)