Say you have a scientific theory that talks about 'electrons.' No one has ever observed an electron, so there's a question as to the origin and content of your concept 'electron'. What do you mean by the word? Ramsey and Lewis propose that the meaning of the term 'electron' is given implicitly by the scientific theory, with all its assertions about electrons. Electrons are those things about which all those statements are true.
But things are not as simple as it sounds, because some of those statements are also statements about other theoretical entities that we haven't observed and theoretical properties such as 'charge' and 'spin'. For instance, "Electrons attract protons" and "Electrons have negative charge."
So take a sentence like "There's an electron in the sink." This means something like this: "There exist some properties P1, P2, ... Pn (note: one for every theoretical property involved in your total scientific theory, and 'electronhood' counts as such a 'property') such that ... (note: fill this in with a statement of the scientific theory, but with P1, ... Pn substituted for the theoretical terms 'charge', 'is an electron', etc.), and there is something in the sink that has P1."
Example: Suppose there are only three principles in our scientific theory about electrons:
"There are properties P1 and P2 such that (things with P1 also have P2, and things with P2 tickle you, and things with P1 cause lightning), and there is a thing with P1 in my pocket."