The bar graph shows the level of antibodies made in response to the vaccine when given to primates. Those antibodies neutralize the virus. The Novavax approach seems to get a much higher response than the other three.
I am roughly familiar with stuff like this after taking second semester Sophomore biology in college (disease and immunity). We had New Zealand white rabbits and inoculated them weekly with serratia marcescens (harmless to the rabbit), took blood samples, and ran the titers to demonstrate the weekly increase in antibodies.