When a spider appears in our nightmares, it is usually a warning, in the sense that we could fall into a trap. This is so, since the sign comes from the behavior of spiders, so spiders often make cobwebs that are a type of trap, through which they capture certain insects, to feed; hence the comparison.

So starting from that point, the details of the dream can tell us what type of trap it is, for example: If we see spiders getting out of bed, it means that the trap is related to sex or the couple (Since the bed is related to issues of sex and the couple); if it is a giant spider, it represents what we must overcome, so to avoid falling into traps, we must be cautious and take greater precautions (Since giant things represent everything that we must overcome and the spider a trap).