Le baudrier est attaché à la corde.

Le means the, while un means a/an.

So le baudrier means the harness, referring to a specific harness that is already known in the context.

• Le baudrier = the harness
• Un baudrier = a harness

French often uses the definite article where English would also use the.

Baudrier is a masculine noun. In this sentence, it usually means a harness, especially in a climbing or safety context.

Depending on context, French can also use harnais for harness, and in many modern contexts harnais is more common. But baudrier is still a correct and commonly understood word, especially for a climbing harness.

Est attaché literally means is attached.

This is formed with:

• est = is (from the verb être, to be)
• attaché = attached

So the sentence describes a state or uses a passive meaning:

• Le baudrier est attaché à la corde = The harness is attached to the rope

By contrast, attache on its own would usually be a form of the verb attacher meaning attaches:

• Il attache le baudrier = He attaches the harness

Attaché is the past participle of attacher (to attach).

Here it is used with être:

• est attaché = is attached

Because baudrier is masculine singular, the form stays attaché.

If the noun were feminine singular, it would become:

• La corde est attachée...

If it were masculine plural:

• Les baudriers sont attachés...

If it were feminine plural:

• Les cordes sont attachées...

In French, the expression is normally attaché à when something is attached to something else.

So:

• attaché à la corde = attached to the rope

Other prepositions mean different things:

• avec = with
• de = of/from

For example:

• attaché avec une sangle = attached with a strap
  (this tells you the means or tool)
• attaché à la corde = attached to the rope
  (this tells you what it is connected to)

Because corde is a feminine singular noun:

• la corde = the rope

After the preposition à, you get:

• à la corde = to the rope

Compare:

• à le becomes au before a masculine singular noun
  
  • au mur = to the wall
• à la stays à la before a feminine singular noun
  
  • à la corde = to the rope
• à l’ is used before a vowel sound
  
  • à l’arbre

A rough pronunciation guide is:

luh boh-dree-ay eh-tah-shay ah lah kord

A few notes:

• le is often pronounced lightly, almost like luh
• baudrier sounds roughly like boh-dree-ay
• est attaché links smoothly in natural speech
• corde ends with a pronounced d-like sound? No: in standard French, the final e is silent, so it sounds roughly like kord

A more natural flow would be something like:

L’baudrier est attaché à la corde

The e in le is often reduced in fast speech.

Usually it describes the resulting state: the harness is attached.

In many cases, French does not sharply separate:

• a true passive meaning (has been attached)
• and a state (is attached)

So in practice, Le baudrier est attaché à la corde most naturally tells us the current situation: the harness is connected to the rope.

Actually, the word order here is very close to English:

• Le baudrier = The harness
• est attaché = is attached
• à la corde = to the rope

So French and English match quite neatly in this sentence:

• The harness is attached to the rope
• Le baudrier est attaché à la corde

This is one reason the sentence is fairly easy for English speakers to understand once the vocabulary is known.

Yes, you could, but it is not exactly the same nuance.

• attaché à = attached to, fastened to
• relié à = connected to, linked to

So attaché suggests a more physical fastening.
Relié is broader and can mean physical or abstract connection.

For climbing or equipment, attaché à is often the more natural choice if you mean it is fastened onto the rope.

Baudrier is masculine, which is why the sentence uses:

• le baudrier
• attaché (not attachée)

Unfortunately, you often cannot know a noun’s gender just by looking at it. You usually need to learn it together with the article:

• le baudrier
• la corde

That is the best habit in French: memorize nouns with le or la, not by themselves.