Extension means by which we can think that this file is related to this language or application.
Extension of Html is .html
Extension of any Windows application is .exe
Extension of Python is .py
Extension of php is .php
and so on....
I want to try to explain what tags are. I noticed you asked about it in your question tags.
Tags in this Q&A forum is used for (among which):
To clarify context of the question, by language or subject. For example when a question asks about 'list' in Python language, the recommended tags are 'Python list'. If the question is related to SoloLearn as platform and/or the features she provides, then 'SoloLearn' is good candidate as tag.
Tags are used by SoloLearn search engine when finding threads under similar topics. This is done by matching words in search term against the threads' tags. A match means the thread is possibly included within the search result.
Use of irrelevant words in tags will diminish search accuracy, and performance. Not to mention that a thread with irrelevant tags won't become easily searchable because tags simply don't match search terms.
There are probably other uses of tags that I just don't know. But hopefully this gives you an idea what tags are 👌
Derios Santen when you try to understand it grammatically, it will be complicated, just keep in mind that extension means (.something) just like 🅰🅹 - Pʀᴏᴅᴜᴄᴛ Mᴀɴᴀɢᴇʀ said before.
Programming languages have their own grammar far away from English grammar.