Devices for encoding and decoding - that is, changing lists of symbols into other lists of symbols.
The word “symbol” and “encoding” here are used in their most stupid and brutal sense. Because the number of symbols is countable, we just use numbers for all the symbols of the world. To “encode” something means to transform one list of symbols into another.
Computers as we understand them began as a thought experiment about the limits of this sort of thing: they emerge from a proof about what we can’t use them for.
Like a lot of first papers, the first paper in computer science is relatively easy for lay-people to read: there weren’t any computer science specialists around at the time it was published to read it. You can read it yourself of it here.