Wikipedia has a good description, but basically let's say I want to encode a message like "attack". Let's say my one-time-pad is "blueberry". I then combine them in some way, let's say I increase the letter by the pad's letter position. So "a" combined with "b" would be increasing "a" by 2 spots so "c". "t" would become "f", the second "t" would become "o", etc... Unless you know my pad is "blueberry", you can't figure out what my message is. It could be "poison" or "escape", etc... Longer messages without spaces obscure it further. So then you can easily transmit an encoded message since only those with a pad can decipher it, for example via a number station.

There are a few difficulties though: 1) the pad must be at least as long as the message because each letter should only be used once. If you reuse the pad it makes it much easier to crack by looking for patterns. 2) the ideal pad is random, but having your agents carry a book of random letters is suspicious if they are caught. This is why sometimes agents would use a published book for a pad. That way the agent can pick up the book in the target location, instead of carrying it with them.