Meaning of plug the gap

[Mick]

What does mean "to plug the gap" ????

[Alan]

Plug the gap in a figurative sense means put in something that is missing. There could be something missing in your education - something you did not learn at school - and in later life you decide to find out about it and so plug the gap in your learning.
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English as a Second Language
You can read my ESL story Prepositions