Learn Hardcore Icelandic: Netnámskeiðið er ókeypis í dag.

In Icelandic the definite article (the) is usually attached to the end of the noun.
So netnámskeið = online course, and netnámskeiðið = the online course.
Here -ið is the definite article for a neuter noun in the singular (in the nominative/accusative form).

How do I know netnámskeið is neuter, and why does that matter here?

Gender is a property of the noun in Icelandic (masculine/feminine/neuter) and often has to be learned with the word. Námskeið (and compounds ending in it, like netnámskeið) is neuter.
It matters because:

the attached definite article changes by gender (-inn, -in, -ið, etc.)
adjectives sometimes agree with the noun (though ókeypis is a special case; see below)

Why is the sentence Netnámskeiðið er ókeypis í dag and not Netnámskeiðið er ókeypist (with adjective agreement)?

Many Icelandic adjectives agree in gender/number/case (e.g., góður/góð/gott). But ókeypis is commonly used as an indeclinable adjective (it typically doesn’t change form).
So you say er ókeypis, not ókeypist.