هذه دراجة مستعملة.

Because دراجة (bicycle) is a feminine noun in Arabic, so the demonstrative must also be feminine.

• هذه = this for feminine nouns
• هذا = this for masculine nouns

So:

• هذه دراجة = this is a bicycle
• هذا كتاب = this is a book

In Arabic, the verb to be is usually not expressed in the present tense in sentences like this.

So Arabic says:

• هذه دراجة مستعملة

  Literally: this bicycle used

But the natural English meaning is:

• This is a used bicycle.

This kind of sentence is called a nominal sentence in Arabic.

If you wanted was or will be, Arabic would express that differently.

Because in Arabic, adjectives normally come after the noun they describe.

So:

• دراجة مستعملة = a used bicycle
• literally: bicycle used

This is the normal word order in Arabic:

• noun + adjective

Compare:

• كتاب جديد = a new book
• سيارة كبيرة = a big car

Because Arabic adjectives must agree with the noun they describe.

Since دراجة is feminine singular, the adjective must also be feminine singular:

• دراجة مستعملة = feminine noun + feminine adjective

If the noun were masculine, the adjective would be masculine too:

• كتاب مستعمل = a used book

So the -ة here shows feminine agreement.

Because the phrase is indefinite: a used bicycle, not the used bicycle.

In Arabic, adjectives match the noun in definiteness too.

So:

• دراجة مستعملة = a used bicycle
• الدراجة المستعملة = the used bicycle

Both words are indefinite in your sentence, so neither one takes ال.

Yes, depending on context, it can be understood in both ways.

However, the most natural reading for many learners is:

• This is a used bicycle.

Since مستعملة often means used / second-hand, the sentence commonly describes the bicycle as not new.

In the right context, it could also mean:

• This bicycle is used
  as in it has been used, but used bicycle or second-hand bicycle is usually the clearer interpretation.

مستعملة is a form derived from the verb استعمل (to use). In this sentence, it functions as an adjective, meaning used.

You can think of it as similar to a passive participle in meaning:

• مستعمل = used (masculine)
• مستعملة = used (feminine)

So:

• كتاب مستعمل = a used book
• دراجة مستعملة = a used bicycle

A common pronunciation is:

• hādhihi dirāja mustaʿmala

A few notes:

• هذه = hādhihi
• دراجة = dirāja
• مستعملة = mustaʿmala

The symbol ʿ represents the Arabic letter ع, a sound English does not have.

In normal reading and speech, short case endings are often not pronounced, so learners usually hear and say the sentence this way.

It would typically be written as:

• هٰذِهِ دَرّاجَةٌ مُسْتَعْمَلَةٌ

In careful grammar terms:

• دَرّاجَةٌ has -un because it is indefinite
• مُسْتَعْمَلَةٌ also has -un because it is an indefinite adjective matching the noun

But in everyday printed Arabic, these final short vowels are usually omitted, so you normally see:

• هذه دراجة مستعملة

Because هي is not needed here.

The simple and natural Arabic sentence is:

• هذه دراجة مستعملة

Adding هي would usually be unnecessary in this kind of basic identification sentence. Arabic often prefers the shorter nominal structure.

So for a learner, the best model is:

• هذه دراجة
• هذه دراجة مستعملة

You would make both the noun and adjective definite:

• هذه الدراجة المستعملة

That means:

• هذه = this
• الدراجة = the bicycle
• المستعملة = the used one / used

So:

• هذه دراجة مستعملة = This is a used bicycle.
• هذه الدراجة المستعملة = This is the used bicycle.

That matching of definiteness between noun and adjective is very important in Arabic.