Ses boucles d’oreilles sont moins lourdes que celles d’hier.

Ses is a possessive adjective: son / sa / ses = his / her / its.

• ses boucles d’oreilles = his/her earrings
• It agrees with the thing owned, not with the owner.
  
  • ses is plural because boucles d’oreilles is plural.
• ces boucles d’oreilles would mean these earrings (demonstrative, not possessive).
• les boucles d’oreilles would mean simply the earrings, with no idea of possession.

So ses tells us they belong to “him/her” (context decides which).

In French, une boucle d’oreille is one earring, and des boucles d’oreilles is earrings.

You normally wear two, so people usually speak in the plural:

• Ses boucles d’oreilles = her/his earrings (both ears)
• If you really mean only one: sa boucle d’oreille = his/her earring

The expression boucles d’oreilles is literally ear loops and is almost always used in the plural when you talk about someone’s earrings.

The adjective lourd / lourde (heavy) must agree with the noun boucles d’oreilles.

• boucle is feminine: une boucle
• boucles d’oreilles is feminine plural

So:

• singular feminine: une boucle lourde
• plural feminine: des boucles lourdes

In the sentence, lourdes is feminine plural to match ses boucles d’oreilles.

Moins … que is the standard structure for less … than:

• moins lourd(e)(s) que = less heavy than

Pattern:

• sujet + verbe + moins + adjectif + que + complément
• Ses boucles d’oreilles sont moins lourdes que celles d’hier.
  = Her earrings are less heavy than yesterday’s (ones).

You can use the same pattern with other adjectives:

• Il est moins grand que son frère. = He is less tall than his brother.
• C’est moins intéressant que le film d’hier. = It’s less interesting than yesterday’s film.

Celles is a demonstrative pronoun: it stands for a previously mentioned feminine plural noun.

• celles = the ones (feminine plural)
• It replaces les boucles d’oreilles from the comparison:
  … moins lourdes que celles d’hier
  = … less heavy than the ones from yesterday.

Choice of form:

• masculine plural: ceux
• feminine plural: celles
• masculine singular: celui
• feminine singular: celle

We use celles because boucles (d’oreilles) is feminine plural.

Forms like celles-ci / celles-là add “these / those”, but here we don’t need here/there, just the ones.

In French, you cannot normally say les d’hier to mean yesterday’s ones. You need a pronoun that can stand alone.

• celles d’hier = the ones from yesterday / yesterday’s ones
• celles replaces les boucles d’oreilles
• d’hier specifies from yesterday

Compare:

• Ses boucles d’oreilles sont moins lourdes que celles d’hier.
  ✔ natural and correct
• Ses boucles d’oreilles sont moins lourdes que les d’hier.
  ✘ incorrect in standard French

D’ is a contracted form of de before a vowel sound.

In celles d’hier:

• de roughly means of / from
• hier means yesterday
• d’hier together means from yesterday or yesterday’s

So celles d’hier is literally the ones of yesterday → yesterday’s ones.

Grammatically you can say:

• sa boucle d’oreille = his/her earring (one single earring)

But in this context, we are comparing the pair of earrings worn today with the pair worn yesterday, so we naturally use the plural:

• Ses boucles d’oreilles sont moins lourdes que celles d’hier.

If you were talking about only one earring (for example, someone who only wears one), you could say:

• Sa boucle d’oreille est moins lourde que celle d’hier.
  (His/her earring is less heavy than yesterday’s one.)

In French, the comparative moins normally goes directly before the adjective:

• moins lourdes (less heavy)
• moins intéressants (less interesting)
• moins cher (cheaper / less expensive)

The order sont moins que lourdes would sound very unnatural and is not how the comparative works.

Correct pattern:

• être + moins + adjectif + que
  Elles sont moins lourdes que celles d’hier.

Yes, you could say:

• Ses boucles d’oreilles sont plus légères que celles d’hier.
  = Her earrings are lighter than yesterday’s (ones).

Both are grammatically correct:

• moins lourdes = less heavy
• plus légères = lighter

In most contexts they mean the same thing, but there can be a small nuance:

• moins lourdes focuses on the reduction of heaviness (maybe they were very heavy before).
• plus légères emphasizes lightness as a positive quality.

In everyday speech, both are perfectly natural.