Details

link must be an active LinkObject, as returned by functions like LinkLaunch or Links.
You can use LinkWrite[link,Unevaluated[expr]] to write expr to the link without evaluating it.
The head of expr will often be a packet that specifies how expr should be processed by the program which receives it.
When LinkWrite is used to send data to a Wolfram Language kernel, EnterTextPacket["string"] enters the text of an input line, and EvaluatePacket[expr] sends an expression for evaluation.

Examples

Launch a subsidiary Wolfram Language kernel process:

Wolfram Language code: link = LinkLaunch[First[$CommandLine] <> " -wstp"]

Read the first packet:

Wolfram Language code: LinkRead[link]

Write the expression 2+2 to the link without evaluating it:

Wolfram Language code: LinkWrite[link, Unevaluated[2 + 2]]

Read the result:

Wolfram Language code: LinkRead[link]

Close the link:

Wolfram Language code: LinkClose[link]

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