In French, we cannot use a noun without an article.

You’ll always find an article before a noun.
As you already know all nouns have a gender in French. Either masculine (nm. in the dictionary)  or feminine (nf.)

The gender of the noun defines the article that need to be used:

Le for masculine singular noun -> Le tableau (the board)
La for feminine noun -> La table (the table)                          L’ before ‘h’ and vowels -> L’ami
Les for plural (either feminine or masculine) -> Les étudiants / Les étudiantes

Un for masculine singular -> Un stylo (a pen)
Une for feminine singular -> Une chaise (a chair)
Des for plural (either fem or masc) -> Des élèves (some students or students)

So now what’s the difference between Le/La/Les & Un/Une/Des?
Le/La/Les are called « definite articles » they define something specific
Un/Une/Des are called « non-definite articles » they define something not specific, random

See the picture below for more explanation:

les_articles_d_finis_et_ind_finis

And with a video of Mafalda you cannot not get it right!

 

Bon apprentissage !