When encountering another vessel in darkness or reduced visibility, what is indicated by the presence of only a visible white light?

The vessels use three main colors (white, green and red) where each color alone, combination of two colors as well as the three colors together each has a certain meaning. This meaning is sort of an international code that is agreed all over the globe.

So, based on this code, the presence of only visible white light means that your vessel might be overtaking another vessel. In this case, you should give way to either sides. The other vessel should standby until an action is taken from your end.