Transposing (or transposition) means changing the key of a piece of music. By using the system of roman numerals it is very easy to transpose a chord progression from one key to another. The same system is also useful for communicating progressions and parts to other musicians.
The following example shows a simple keyboard part in the key of C, based on the chords C. F and G. This progression can be described as I IV V.
To transpose the previous part to the key of G, you need to know chords I IV V in that key (G, C and D). Here is the same part transposed to the key of G.