What this means is that 50% is random. Wearing a mask versus not wearing a mask in this study gives you a 50% chance of getting sick. In other words, equal numbers of mask wearers and non-wearers got sick. It's a coinflip. Random. In other words, no effect. If it were less than 50% that means that wearing a mask gets you sick more often. If it were "greater than 50%," then wearing mask would reduce your chances of getting ill. However, they couldn't show that it was any greater or less than 50%. Random. Thus it makes no difference. Don't worry. It is confusing language. But in statistics this is how this sentence is interpreted.