We're not referring to the metro gap but the gap between how English is written and how it is pronounced. Knowing how to spell a word in English is no guarantee for how it sounds... and vice versa! English is not a so-called "phonetic" language like Spanish. Here are some examples of sometimes chaotic English spelling.

• Read can be [ri:d] or [red] and [red] can be read of red.
• English is strange sometimes, but it can be understood through tough thorough thought, though (note: you may want to wipe your desk at this point).
• Ghoti is a funny spelling of an existing word. Can you find it? Think of the words laugh, women and station. Note that it is often wrongly attributed to George Bernard Shaw.
• The word queue is just a Q followed by four silent letters patiently waiting for their turn.

If you are more into romantic stories than spelling, you will love this story behind Mind the Gap.