When I was a newbie, I'd I often forget the difference between a KPI and an OKR, so let's start with that.

OKR = Objectives and Key Results

A popular leadership process for setting, communicating and monitoring quarterly goals and results in organizations.

Intended to unite a company, team, and individual with common goals.

OKRs consist of a list of 3-5 high-level objectives.

Under each objective, there should be 3-4 measurable results, not more. OKR results can be based on growth, performance, revenue or engagement. Often they are numerical, but they can also show if something is done or undone, so a binary 0 or 1. These can also be milestones.

KPI = Key Performance Indicator

Key performance indicators determine specific factors needed to achieve success. These are measures that must be watched. They are not goals, but they can be associated with goals. They get specific, and they must be measureable. (Although, once a measure becomes a goal/target–you must question it's validity relevance.)

KPIs are important, and few.

There may be measures we track, but only few KPIs.

A few KPI examples:

Financial

Accounting costs
Accounts payable turnover
Actual expenses
Budgeted expenses
Gross profit
Gross profit margin
Net income
Total sales
Variable costs

Customer Service

Agent's full-time employees (FTEs) as percentage of total call center FTEs
Average after-call work time
Average number of calls/service request per handler
Average queue time of incoming phone calls
Email backlog
Number of complaints

Marketing

Ad click-through ratio (CTR) clicks ÷ impressions
Brand credibility
Brand strength
Cost per lead
Response rate

Sales

Actual calls
Average deal size
Call quota
Closed sales
Customer churn ratio

Information Technology

Account create success
Alert-to-ticket ratio
Network availability
Server growth rate
Web server availability
Software development quality

Other

On-time orders
Open orders
Percentage increase in productivity
Process capability

General

Starting fresh? Basic metrics are just fine. Call them vanity metrics. It's fine.

In general, you'll want to understand

Product usage/adoption (Daily/Monthly Average Users)
Churn rate
% of users who take a specific action

These tend to be lagging indiciators. But how do we understand the customer behavior ahead of time, find our leading indicators, and improve the product experience?

How do we actually measure a success feature launch?

How do we measure success? How do we know that our users are appreciating this feature?
How do we know when our user gets frustrated with this feature?
What is the goal for this feature?
Who are our user segments?

But you won't be using these to actually improve your product experience...

So, how do you actually improve the product experience? Measure it. Measure the experience.

Get specific, down to the type of user and their pain points.

Steps

Understanding the measurement’s purpose
Mapping Measurable Results
Designing Meaningful Measures
Building Buy-in to Measures
Implementing Measures
Reporting Performance Measures
Interpreting Signals from Measures

Primary role for a product manager is

Understand the problem
Empathy for the customer, understanding the the technology, etc. etc. etc...
Being able to communicate the problem

Understand the customer's pain points and build measures around them