Before you can craft anything good in Rust, you must first make a blueprint of it first. The only problem is that they are very hard to get, and the game doesn't explain how to even get them in the first place.
To get blueprints, you need to research an item in the Research Table. You can also experiment with the item in a workbench, which gives you a random blueprint.
This is the short answer, so I'll explain this more in detail.
Researching
Blueprints are the only way to create the locked items in the crafting screen. Once you find an item, you can either choose to keep it or turn it into a blueprint. Low level blueprints can also be found in crates, and are really easy to get.
When you learn a blueprint, you’re able to craft that item as many times as you want if you have the proper materials. To turn any item into a blueprint, you need to use a research table.
Each item you research has a certain scrap cost, which ranges from 20 scrap to 500 scrap depending on the rarity of the item. Placing the item inside the table and pressing “Research” will give you the blueprint after a few seconds.
The item you placed inside the research table will be destroyed, but you’ll get a blueprint that you can learn.
Experimenting
Besides the Research Table, you’re also able to perform experiments in workbenches. Doing an experiment will give you one random blueprint that you haven’t learned before. The 3 workbenches each give you a level of blueprint corresponding with the workbench level.
Researching will always give you a blueprint that you haven’t learned, which you can use to your advantage. If you wanted to get more level 3 blueprints, one player could do experiments until they have all the blueprints of a certain level.
Experiments cost a lot of scrap, but they’re worth it if you want better items. Most of the time, it’s cheaper to research an item that you already have than to get it in an experiment. You should always research valuable items as soon as you get them, or else they could be easily lost.
Blueprint Wipes
Every server has its own wipe schedule, and most of them completely wipe the blueprints that players have learned. Pay attention to this before joining a server, or else you may risk having your blueprints wiped when you’re just getting started.