Here is a list of some topics/tools frequently encountered in software development. Resources for learning them are also included. These appear roughly in decreasing order of importance in my opinion. They are easy and you can learn them in a short time. They have also been rated on the basis of difficulty by Eklavya Sharma, the original author of this article. This rating is relative, so even a 5-star rating is not very hard. Time to learn a topic is roughly proportional to difficulty. It's okay to be lazy and learn them only when you need to.
HTML is used to write web pages. It is a very common text-based format for creating rich text.
To be a frontend or backend developer, you must know HTML. CSS is optional unless you are a frontend developer.
Resources:
Difficulty: ★ ★ ★ ☆ ☆
Git a Version Control System. It is mainly used for collaborating on a project with a team.
Resources:
Difficulty: ★ ★ ★ ★ ☆
A text-based format for writing rich-text documents. Used on Wikipedia, Github, Slack and many other places. This page has been authored in markdown.
Resources:
Difficulty: ★ ☆ ☆ ☆ ☆
JSON (JavaScript Object Notation) is a lightweight data format. Used in web development and other places.
Resources:
Difficulty: ★ ☆ ☆ ☆ ☆
A regular expression (abbreviated regex or regexp) is a sequence of characters that define a search pattern, mainly for use in pattern matching. Used in text editors' find-and-replace, web and software development, sometimes competitive programming and many other places. It is supported in almost all programming languages.
Resources:
Difficulty: ★ ★ ★ ☆ ☆