The slang term “the devil is in the details” has a number of different senses. All of the meanings for the term boil down to the fact that it is often the small details of something which make it difficult or challenging. These details can prolong a task, or foil an otherwise straightforward dealing. Like many proverbs which involve the devil, it is meant to sound a note of caution. It may also be used to excuse or explain the obfuscation of an otherwise very simple project or task.
In one sense, one might say “the devil's in the details” to refer to very small but ultimately important components of a larger task. For example, performing a scientific experiment in a laboratory is a hugely involved task which can sometimes be highly dangerous. A small error at the beginning can translate into a useless experiment at the end, so experimenters are reminded that the devil's in the details. This reminder is intended to encourage the scientists to check their work, and to be thorough and careful in the lab.