It is an SEO know-how. It shows you how good a keyword is for bringing traffic to your site. Or, putting it simply, how much positive effect keywords have on your paychecks.

The Keyword Effectiveness Index (KEI) compares the count result (number of times a keyword is searched within a search engine) with the number of competing web pages to pinpoint which keywords are most effective for your campaign. In combination with some common sense, this is a great tool to select high conversion key phrases you want to target.

KEI was first introduced by Sumantra Roi. He said that KEI was a measure of the effectiveness of a keyword for your web site. The KEI formula is based on three axioms:
(1) the KEI for a keyword should increase if its popularity increases; (2) the KEI for a keyword should decrease if it becomes more competitive. Competitiveness is defined as the number of sites which Google displays when you search for that keyword using exact match search (i.e. you should use quotes around the keyword);
(3) if a keyword becomes more popular and more competitive at the same time such that the ratio between its popularity and competitiveness remains the same, its KEI should increase.

It is the calculation of the frequency/occurrence of a one-word phrase combination. It is an SEO-related topic. It is represented by the number of keywords in every 100 words.

There is no a correct answer to that but in general, a rule would be to keep in between 2% and 3%

Keyword research is when people use keywords to find and research actual search terms that people enter into search engines.
The knowledge about these actual search terms can help inform content strategy or marketing strategy overall.

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Google’s search results often display information sourced from the Knowledge Graph, a vast database containing billions of facts about people, places, and things. The Knowledge Graph enables Google to answer factual questions like “How tall is the Eiffel Tower?” or “Where were the 2016 Summer Olympics held?” I see the Knowledge Graph as a powerful tool that helps Google identify and present accurate, publicly known information that users find valuable. The goal of Google is to ensure that this factual content is easily accessible when it’s most relevant to the searcher’s intent.

As per google: “To help ensure knowledge graph and similar displays are a helpful experience for everyone, we have systems in place to prevent showing those that are in violation of  Google Search’s overall policies or these policies for Search features:
Advertisements
Dangerous content
Deceptive practices
Harassing content
Hateful content
Manipulated media
Medical topics
Regulated goods
Sexually explicit content
Terrorist content
Violence & gore
Vulgar language & profanity

Knowledge panels are information boxes that show up on Google when you search for specific entities like people, places, organizations, or things that are part of the Knowledge Graph. These panels are designed to provide you with a quick overview of essential information about a topic based on Google’s interpretation of available content across the web.

Knowledge panel content is automatically generated, pulling information from various online sources. Sometimes, Google collaborates with data partners who offer expert data on particular subjects, such as movies or music. This data is then combined with information from other public web sources to create a comprehensive snapshot.

If you are the subject of or official representative of an entity depicted in a knowledge panel, you can claim this panel and suggest changes. More information about these processes are available in the following articles:
Get verified on Google
Update your Google knowledge panel

A key performance indicator (KPI) is a metric that helps you understand how you are doing against your objectives.

Create a list of KPIs to monitor and optimize. We shouldn’t select more than 3 KPIs (one is optimum) in parallel and always we need not create conflicting KPIs. For example, if driving traffic is important, driving sales is not possible to become equally important. To elaborate here, to get the traffic KPI you may have to “open up” a bit of the funnel to see who would click on your banners. This doesn’t necessarily mean that those visits would convert into customers.

Very often you may see the same meaning using the terms OKR or North Star Metric.