A codon is basically a "word" in the genetic code.
Let's start at the base. A base is the smallest unit of information in genetics. It is a chemical represented by either A, T , C, G or U.
These genetic "letters" are combined into "words" which are called codons. What is useful is that each codon is only made of 3 bases. You'll have CCT, GAT, TTA etc. So what do these words represent? Well they can be instructions, like START or STOP, but usually they represent an amino acid, the LEGO building blocks of proteins. There are around 20 AAs but a bit of maths tells us we have a lot more codon combinations than AAs. This means we have some synonyms where several codons can represent one AA.
To take the analogy further, the codons combine into "sentences" and then "chapters". The chapters are a gene, usually instructions for a complete protein, but some of the sentences are useless and will be ignored when translating into a protein.
Then we have the "Book" which is the chromosome. The reason I like to think of it as a book is that at this point it is a contained unit, a massive molecule. You can separate up your books and even lend one to a friend, but you can't do that with a chapter unless you destroy the book.
Finally you have you "library" or genome. It's not a big collection, 46 books in total, and in fact you have 2 copies of each. It's like inheriting 2 sets of encyclopaedias from your parents. They both have the same information, it is just explained in different ways.
And there is my book analogy of genetics. It's not perfect but works when used on small children.
