Stardew Valley is a farming game with a story that encourages lots of fishing, mining, and monster combat on the way to becoming a rich farmer. There are few set deadlines and very little pressures besides the goals you set for yourself. There is no AI to compete with and the wilderness provides only the most minor of threats to your progress. There are a wide variety of ways to make money - some that are labor intensive, some that are not. There are many ways to spend money but few ways to actually waste it.
There is a lot of open-endedness in the game; you can develop your farm into a seasonal monoculture or a diversified bit of everything. You can spend lots of time mining or do a lot of processing your produce into valuable goods.
There is a very wide variety of secrets and surprises to uncover - and if you undertake the challenge of restoring the Community Centre by collecting all the diverse items required, you will have to uncover a lot of them. Otherwise you can decline much of that and bypass the diversity of the main story, for a price, by becoming a regular customer of the Joja Corporation.
There are goals but there is no victory condition as such; also no loss condition. There is no “game over.”
Everything is kid-friendly; you get to name every animal, and they are valued for their daily produce. The only meat in the game is fish.
This guide explains a lot of basic details, but is a strategy guide, not a reference guide. For reference purposes, a guidebook is likely useful, or a website like the Official Stardew Valley Wiki. In this guide the focus is on what you should do, and why, whereas a wiki will tell you about everything you could do, with less emphasis on evaluation, decision making & tactics.