However thoughtfully you group your photos in folders, there'll be times when you're searching for specific categories of image that don't fit in that folder structure. So it's a worthwhile time investment to add as many tags as you can. This will be enormously helpful in finding images in future.
This process is quite similar adding hashtags on social media platforms like Instagram, or in a stock photo library. The main difference is that you're adding tags that you, rather than others, would be likely to search for.
Include everything from descriptive words ('nature', 'outdoors', 'snow' etc) to those relating to the image's mood ('happy', 'gloomy', 'peaceful') to technical aspects ('bokeh', '50mm', 'macro'). Again, this sounds like a lot of work, but photo organizer software can help to automate this process.