A library with a specific purpose
Placebo provides shared code used by other mods. It is useful when a selected mod release declares it as a dependency. Installing a library does not give you the gameplay of the mods that use it, so begin with the content mod or modpack you actually want to play.
Read the dependency list first
Open the file details for that mod release and check required dependencies. Record Minecraft, loader and dependency version ranges together. A dependency that applies to one historical release might differ in another, which makes the exact file more useful than a general mod-name search.
Confirm the result
Launch the intended profile and inspect the loader's mod list. Seeing Placebo there confirms discovery, while successful startup confirms the dependency checks passed. If there are still errors, read the requested version range rather than adding random newer jars.