Cămașa pe care o port astăzi este ușor de călcat.

In Romanian, the definite article is not a separate word like in English; it attaches to the end of the noun (this is called the enclitic definite article). For feminine singular nouns ending in -ă, replacing the ending with -a marks the definite form:
• cămașă → cămașa (the shirt).

Pe care is the accusative relative pronoun. When the relative pronoun replaces the direct object, Romanian requires pe + care:
• pe marks a definite direct object.
• care means which/that.
So pe care = that as the object of port.

O is the 3rd person feminine singular direct object pronoun, referring back to cămașa. In Romanian, object pronouns normally precede the verb:
• o port = I wear it.

Ușor de călcat is a fixed adjectival pattern expressing easy to iron:
• ușor stays in the base (masculine singular) form, without agreeing in gender/number.
• de + infinitive (călcat) shows the action.
This construction does not change to match the noun’s gender.

De links the adjective ușor with the infinitive călcat (from a călca). Together they mean easy to iron. Similar patterns:
• ușor de citit = easy to read
• greu de găsit = hard to find

Adverbs of time like astăzi are flexible in Romanian. Placing astăzi after o port focuses on today’s action:
• astăzi o port (today I’m wearing it)
• o port astăzi (I’m wearing it today)
Both orders are correct; the nuance changes slightly but the meaning stays the same.

You can use a subordinate clause with să + subjunctive:
• Cămașa pe care o port astăzi este ușor să o calc.
Here să o calc literally means to iron it. It’s a valid alternative, though ușor de călcat is more idiomatic.