To read between the lines is to look carefully at what is written in order to infer truths which are not explicitly stated. We call this reasoning, and God invites to use it as we seek Him (Is 1:18), employing logic to expand from what is explicitly revealed to see what some might consider hidden, yet if one is paying attention and thinking deeply it becomes obvious.
To illustrate, the arrow in the FedEx logo may not be apparent until someone points it out; but once you see it you can’t stop seeing it. The arrow isn’t exactly hidden, but it isn’t exactly there either.
To see it you must look between the E and x at the resulting white space connecting them, which is really nothing by itself: the mere juxtaposition of the letters reveals a shape implied by what surrounds it, and this insight enhances the logo, making an impression which creates additional value.
There are many truths like this in Scripture; what is explicitly stated in the text often implies priceless truths which remain unwritten. We may consider what is unspoken, which we might think ought to have been spoken, or which is certainly implied by what is stated, to learn more about God and His ways.
For example, when Paul is meditating in De 25:4, “Thou shalt not muzzle the ox when he treadeth out the corn“, he infers a principle for supporting Christian workers. (1Co 9:9-10) Paul reasons from the general context of scripture that God isn’t particularly concerned about the feelings of an ox, so He must be providing a general instruction in how we’re to treat those called serve in ways which make it difficult for them to earn a living in the traditional sense.
We often see this kind of reasoning explicitly stated in Scripture with the phrase how much more; when God shows us how to address the relatively unimportant, He expects us to reason similarly about more important yet related concerns. For example, if the saints shall judge angels, how much more are they qualified to judge temporal matters? (1Co 6:3) If we expect earthly parents to care for their children, how much more should we expect God to care for us? (Mt 7:11) If animal sacrifices sanctify the physical man, how much more shall the blood of Christ sanctify the spiritual man? (He 9:13-14)
We should certainly be careful when looking at the white spaces in scripture, but they’re indeed present and we should be on the lookout for them, meditating both on what’s explicitly written and prayerfully considering what’s implied.